Device, method and system for monitoring pressure in body cavities

ABSTRACT

The present invention relates to a system and method for digital sampling, quantitative analysis and presentation of pressures in a body cavity. The invention also relates to a portable apparatus for monitoring, sampling and storing pressure and a software for analysis of pressures. The invention includes an algorithm for analysis and presentations of pressures and a software for performing the analysis. The computer software may be integrated in the portable apparatus and in a variety of systems. The software provides different quantitative presentations of pressure curves as a matrix of numbers of intracranial pressure elevations of different levels and durations and a matrix of numbers of single pulse pressure waves with preselected characteristics. The parameters may be standardised according to recording time and heart rate variability. The data may be presented in different ways, both on-line and off-line after pressure monitoring.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method, an apparatus, a systemand a computer program product for monitoring and analyzing the pressurewithin a cavity in a patient, and more specifically, but not by way oflimitation, apparatuses, methods and systems for monitoring andanalyzing intracranial pressure and blood pressure, or pressures inother body cavities (e.g. cerebrospinal fluid space). The inventionincludes an apparatus for sampling, recording, storing and processingpressure measurements, a method, a system and computer software forquantitative analysis of the pressure. The invention aims at providing atechnical solution for digital pressure monitoring in patients that arefree to move about, as well as a technical solution for comparisons ofcontinuously recorded pressures between patients and within patients.The computer software may be used in the portable apparatus describedhere or integrated in various computer systems or vital sign monitors.

[0003] 2. Related Art

[0004] The clinical use of intracranial pressure monitoring was firstdescribed by Janny in 1950 and Lundberg in 1960. During the last twodecades the clinical application of continuous intracranial pressuremonitoring has increased dramatically after the introduction of newintracranial pressure microtransducers in the 1980's. So-called infusiontests were introduced in 1970 by Katzman and Hussey. Infusion tests maybe performed in a variety of ways, basically by measuring pressures incerebrospinal fluid while a fluid is introduced into the cerebrospinalfluid cavity. Intracranial pressure monitoring has been most extensivelyused in the monitoring of critically ill patients with brain damage(e.g. due to head injury or intracranial haemorrhage). It is wellrecognised that abnormal increases in intracranial pressure may lead tobrain damage and even death. In these cases a pressure sensor isimplanted within the skull of the patient, the sensor is connected to apressure transducer that is connected to the monitoring system of thepatient.

[0005] Intracranial pressures may be measured by different strategies.Solid or fibre-optic transducers may be introduced into the epidural orsubdural spaces, or introduced into the brain parenchyma. Intracranialpressure also may be recorded directly by measuring pressure in thecerebrospinal fluid, requiring application of catheter to thecerebrospinal fluid space (most commonly in the cerebral ventricles orthe lumbar spinal cavity). During infusion tests the pressure in thecerebrospinal fluid is recorded.

[0006] A number of intracranial pressure sensors and microtransducersare commercially available, both solid and fibre-optic transducers. Themost commonly used invasive transducers include the Codman® Micro SensorICP Transducer (Codman & Shurtlef Inc., Randolph, Mass.) and theCamino®-110-4B (Camino Laboratories, San Diego, Calif.). Others are ICPMonitoring Catheter Kit OPX-SD (InnerSpace Medical, Irvine), Epidyn®(Braun Melsungen, Berlin), Gaeltec ICT/B (Novotronic GmbH, Bonn),HanniSet® (pvb medizintechnik gmbh, Kirchseeon), Medex® (Medex medicalGmbH, Ratingen) and Spiegelberg® (Spiegelberg K G, Hamburg). Themicrotransducers give an analog signal that is sent to the apparatus.Commonly used equipments for intracranial pressure monitoring include:Codman ICP Express (Codman & Shurtlef Inc., Randolph, Mass.) and CaminoOLM 110-4B (Camino Laboratories, San Diego, Calif.). The equipments maybe connected to other monitor systems. The equipments presentlyavailable are developed for on-line intracranial monitoring incritically ill patients staying in the intensive care unit, that ispatients with head injury or intracranial haemorrhage. Intracranialpressure is recorded online, providing the opportunity for acuteinterventions in order to reduce abnormal rises in intracranialpressure. For the individual case, the storing of pressure values foranalysis later on has limited clinical value.

[0007] In not-critically ill patients outside the intensive care unit,continuous intracranial pressure monitoring has been less extensivelyused. Such patient groups include children with potential intracranialhypertension caused by hydrocephalus, craniosynostosis, shuntdysfunction, or other problems. In adults, clinical entities such asnormal pressure hydrocephalus are included. In these patientsintracranial pressure monitoring is performed in awake patients eithersitting or lying in the bed and the intracranial pressure curve isanalyzed off-line after the intracranial pressure monitoring has beenterminated. In these cases the primary object with intracranial pressuremonitoring is to detect abnormal high intracranial pressure and abnormalelevations of intracranial pressure. The results of the analysis may beused in the pre-operative assessment to select patients for surgery(e.g. extracranial shunt treatment, shunt revision or cranial expansionsurgery). Only a few neurosurgical departments perform this type ofintracranial pressure monitoring, and then usually in connection withresearch not the daily clinical activity. There are several reasons forthis situation: Invasive intracranial pressure monitoring provides asmall but definitive risk of complications. It has been very difficultto analyze the intracranial pressure curve in a reliable and accurateway. Accordingly, it has been difficult to justify a procedure with somerisk of complication when the outcome of the procedure is uncertain.

[0008] The currently available equipments for intracranial pressuremonitoring designed for use in patients within the intensive care unit,do not fulfil the needs for monitoring patients that are not bed-ridden.The apparatuses that are available require the patient to be bedridden,thereby providing a less physiological monitoring of intracranialpressure. The currently available equipments for this type of monitoringare not portable apparatuses that may be carried by the patient. Inparticular, the evaluation of shunt failure patients should be performedin freely moving patients, as over-drainage is a common problem in thesecases. The development of non-invasive pressure transducers, as well asthe development of pressure transducers that may be implanted within ahuman body cavity increases the need for portable apparatuses forpressure sampling and monitoring.

[0009] In the clinical setting, the question may be whether a continuouspressure recording of several hours is normal, borderline or abnormal.Continuous intracranial pressure curves usually are evaluated bycalculation of mean intracranial pressure. With regard to rises inpressure most authors identify so-called pressure waves: Lundberg's Awaves (50-10 mmHg lasting 5-20 minutes), B waves (up to 50 mmHg with afrequency 0.5-2/min), and C waves (up to 20 mmHg with a frequency4-8/min). However, the description of such waves is quite subjective andbased on a morphological description of the waves. Actually the variousauthors differently describe such waves.

[0010] The present invention deals with strategies to analyze singlepulse pressure waves, and make analysis of these waves available to thedaily clinical practice. The fluctuations of intracranial pressure arisefrom cardiac and respiratory effects. The intracranial pressure cardiacwaves or cerebrospinal fluid pulse waves result from the contractions ofthe left cardiac ventricle. The intracranial pressure wave or thecerebrospinal fluid pulse wave resemble the arterial blood pressurewave, that is characterized by a systolic rise followed by a diastolicdecline and a dicrotic notch. In addition, changes in pressuresassociated with the respiratory cycle affect the intracranial pressurewave. The morphology of the intracranial pulse pressure wave depends onthe arterial inflow, venous outflow, as well as the state of theintracranial contents. The single pulse pressure waves of intracranialpressure include three peaks that are consistently present,corresponding with the arterial pulse waves. For a single pulse pressurewave the maximum peak is termed P1 or top of the percussion wave. Duringthe declining phase of the wave, there are two peaks namely the secondpeak (P2), often termed the tidal wave, and the third peak (P3), oftentermed the dicrotic wave. Between the tidal and dicrotic waves is thedicrotic notch that corresponds to arterial dicrotic notch. In thepresent application, the amplitude of the first peak (ΔP1) is defined asthe pressure difference between the diastolic minimum pressure and thesystolic maximum pressure, the latency of the first peak (ΔT1) isdefined as the time interval when pressures increases from diastolicminimum to systolic maximum. The rise time (ΔP1/ΔT1)) is defined as thecoefficient obtained by dividing the amplitude with the latency. Themorphology of the single pulse pressure wave is intimately related toelastance and compliance. Elastance is the change in pressure as afunction of a change in volume, and describes the effect of a change involume on intracranial pressure. Compliance is the inverse of elastanceand represents the change in volume as a function of a change inpressure. Therefore, compliance describes the effect of a change inpressure on craniospinal volume. Elastance is most useful clinically aselastance describes the effect of changes in intracranial volume onintracranial pressure. The relationship between intracranial pressureand volume was described in 1966 by Langfitt and showed an exponentialcurve, where the slope of any part of the curve resembles the rise timeof a single wave (that is ΔP/ΔT or change in pressure/change in volume).The curve is termed the pressure-volume curve or the elastance curve.The horizontal part of the curve is the period of spatial compensationwhereas the vertical portion is the period of spatial decompensation.When elastance increases also the amplitude of a single pulse pressurewave increases due to an increase in the pressure response to a bolus ofblood from the heart. It has, however, not been possible to take theknowledge of single wave parameters into daily clinical practice.

[0011] Another reason for the less widespread use of continuousintracranial pressure monitoring in not critically ill patients is thatthere are still no generally accepted methods for analyzing intracranialpressure. Though there are large amounts of experimental data concerningsingle pulse pressure waves and their relationship to the pressurevolume curve, the clinical application of this knowledge has not beenstraightforward. During continuous intracranial pressure monitoring inclinical practice the single pulse pressure waves are not assessed andused in the decision making. An indirect approach has been Fast FourierTransformation or spectral analysis to assess the frequency distributionof the various waves. Strategies to examine the pressure volumerelationship in a single patient have involved infusion of fluid to thecerebrospinal fluid space or inflation of a balloon, but thesestrategies are invasive, and neither involve assessment of single pulsepressure waves. In the clinical context, methods to explore thepressure-volume relationships or elastance by analysis of the pressurecurve are lacking. There are no strategies that make it possible todetermine accurately where a single patient is on the elastance curve.

[0012] In the intensive care unit, continuous intracranial pressuremonitoring usually presents the pressures as mean pressure in numericalvalues, or as a curve that has to be visually analyzed. Though singlewaves may be displayed on the monitor, strategies to explore trends inchanges of single wave characteristics are lacking. Furthermore,strategies to continuously examine compliance solely on the basis of thepressure curves have not been established.

[0013] Normal mean intracranial pressure has not been defined, anddepends on age. In children most authors consider mean intracranialpressure of 10 mmHg or below as normal, mean intracranial pressurebetween 10 and 15 mmHg as borderline and mean intracranial pressureabove 15 mmHg as abnormal. In adults a mean intracranial pressure of12-15 mmHg or below usually is considered as normal. However, the meanintracranial pressure represents only one facet of an intracranialpressure curve that may include elevations of intracranial pressure ofvarious durations. Obviously, for different intracranial pressure curvesequal mean intracranial pressures may include different proportions ofpressure elevations and depressions. Furthermore, the description ofplateau waves may be inaccurate as the A, B and C waves may bedifferently defined by the various physicians, as the different wavesusually are identified on the basis of the morphology of theintracranial pressure curve. This is illustrated by the fact thatdifferent authors report a large variation in the frequency of B-wavesthat they consider as normal. Attempts also have been made todifferentiate B waves into different types on the basis of themorphology of the waves. Thus, the interpretation of an intracranialpressure curve will be very observer dependent. Since the consequencesof pressure monitoring are so important (surgery or not) accurate andreliable conclusions of intracranial pressure monitoring are needed forthis method to be of interest in daily clinical activity. Similarly,accurate criteria for the normal variation of blood pressure arelacking. The current criteria are wide.

[0014] Strategies to accurately compare pressure recordings (eitherwithin or between individuals) are sparse. With regard to intracranialpressure, mean intracranial pressure may be compared or the distributionof plateau waves (A- or B-waves). With regard to blood pressure,systolic and diastolic pressure may be compared. Nevertheless, thestrategies remain subjective and not very precise. Accurate comparisonsof pressures within individuals might be useful for comparing pressuresbefore and after treatment (for example blood pressure treatment), aswell as detecting changes in pressure trends in patients undergoingcontinuous pressure monitoring. The ability to accurately comparepressure recordings between individuals might be helpful in establishingnormative criteria for pressures within a human body cavity. Forexample, presently it is not possible to compare the pressures of anindividual against a reference curve.

[0015] There is a close relationship between blood pressure andintracranial pressure as the intracranial pressure waves are built upfrom the blood pressure waves. Simultaneous assessment of intracranialpressure and blood pressure provides several advantages, for instance bycalculation of the cerebral perfusion pressure (that is mean arterialpressure minus intracranial pressure). The assessment of cerebralperfusion pressure represents a critical parameter in the monitoring ofcritically ill patients. Assessment of blood pressure per se also has amajor place in daily clinical practice, including both assessments ofdiastolic and systolic pressures.

SUMMARY OF THE INVENTION

[0016] On this background the applicant developed technical solutionsfor monitoring pressures in patients that are free to move about, foraccurate digital sampling and analysis of pressure recordings, as wellas a technical solution for comparing pressure recordings within orbetween individuals.

[0017] An apparatus was developed allowing direct communication betweenthe pressure transducer and a computer (that is add-on to computers suchas medical device computers, vital signs patient monitors, or as astand-alone system for sampling of pressure recordings). Furthermore, anew algorithm for sampling, analysing and presenting pressure recordingswas developed and incorporated in computer software. The computersoftware records, samples, analyses, and provides various outputs of thepressure recordings. The technical solution may be applied to a varietyof pressures such as intracranial pressures (or cerebrospinal fluidpressures), blood pressures, or other body cavity pressures. Invasive ornon-invasive sensors may record pressures.

[0018] According to the invention, the intracranial pressure curve isquantified in different ways. The pressure recordings may be presentedas a matrix of numbers of intracranial pressure elevations of differentlevels (e.g. 20, 25 or 30 mmHg) and durations (e.g. 0.5, 1, 10 or 40minutes), or a matrix of numbers of intracranial pressure changes ofdifferent levels and durations. The pressure recordings also may bepresented as a matrix of numbers of single pulse pressure waves ofcertain characteristics. In this context, elevations are understood asrises in pressure above the zero level that is relative to theatmospheric pressure. An elevation of 20 mmHg represents the pressure of20 mmHg relative to the atmospheric pressure. Pressure changes representthe differences in pressures at different time stamps. A pressure changeof 5 mmHg over a 5 seconds period represents the differences in pressureof 5 mmHg over a 5 seconds measuring period. It should be understoodthat each pressure recording is measured along with a time stamp. Allpressure signals are measured along a recording time. Similar analysiscan be made for blood pressure and cerebral perfusion pressure.

[0019] With regard to sampling, analysis and presentation of singlepulse pressure waves, relative differences in pressures and relativetime differences are computed. The analysis is not relative to the zerolevel or the atmospheric pressure, therefore the results of dataanalysis are not affected by the zero level or drift of zero level.

[0020] By means of the invention used as stated above, the applicant wasable to show in a study including 127 patients that the calculation ofmean intracranial pressure is an inaccurate measure of intracranialpressure. There was a weak correlation between mean intracranialpressure and the number of intracranial pressure elevations. A highproportion of abnormal intracranial pressure elevations may be presentdespite a normal mean intracranial pressure. In another study including16 patients undergoing continuous intracranial pressure monitoringbefore and after cranial expansion surgery, the applicant found thatcalculation of numbers of intracranial pressure elevations of differentlevels and durations in a sensitive way revealed changes in intracranialpressure after surgery. Comparing mean intracranial pressure before andafter surgery did not reveal these changes. Accordingly, this type ofquantitative analysis of the intracranial pressure curve represents afar more accurate and reliable way of analyzing intracranial pressurethan the classical ways of analyzing mean intracranial pressure anddescribing Lundbreg's A, B or C waves.

[0021] With regard to single pulse pressure waves, the inventionprovides measurement and analysis of the following parameters:

[0022] a) Minimum is defined as the diastolic minimum pressure of thesingle wave, or as the valley of the wave.

[0023] b) Maximum is defined as the systolic maximum pressure of thesingle wave, or defined as the peak of the wave.

[0024] c) Amplitude is defined as the pressure difference between thesystolic maximum pressure and the diastolic minimum pressures during theseries of increasing pressures of the single wave.

[0025] d) Latency is defined as the time of the single wave when thesequence of pressures increases from minimum pressure to maximumpressure.

[0026] e) Rise time is defined as the relationship between amplitudedivided by latency, and is synonymous with the rise time coefficient.

[0027] f) Wavelength is defined as the duration of the single pulsepressure wave when pressures changes from minimum and back to minimum,and reflects the heart rate.

[0028] As mentioned in the Related Art section, amplitude, latency andrise in the present invention is referring to the first peak (P1). Thisdoes not represent a limitation of the scope of the invention, however,as amplitude, latency and rise time also may be calculated for thesecond (P2) and third (P3) peaks as well.

[0029] By means of the invention the applicant showed that quantitativeanalysis of characteristics of single pulse pressure waves revealedimportant and new information about the pressures. Both these latterparameters are important for assessment of abnormal pressures. Theapplicant has demonstrated (not published) that parameters of the singlepulse pressure waves analyzed and presented quantitatively, provideinformation about compliance and elastance.

[0030] The quantitative method was developed for various pressures suchas blood pressure, intracranial pressure (subdural, epidural,intraparenchymatous, or cerebrospinal fluid pressure), and cerebralperfusion pressure.

[0031] Furthermore, the quantitative method was developed for offeringdifferent types of data presentations:

[0032] a) matrix presentations of numbers or percentages of single pulsepressure waves with pre-selected characteristics during a recordingperiod,

[0033] b) graphical presentations of single pulse pressure waves withthe opportunity to compare single waves, either between individuals,against a reference material or within the same individual at differenttime intervals,

[0034] c) various types of statistical handling of the data arepossible.

[0035] This invention relates to a method, an apparatus, a system and acomputer program product for recording pressure in a human body cavity(invasively or noninvasively), sampling and processing the recordedpressure signals, performing mathematical analysis, and providingpresentations of the recorded and analysed data (either via monitors,flat screens, or integrated in computer systems).

[0036] One object of the present invention is to provide a technicalsolution for continuous digital sampling of pressures in a body cavitysuch as intracranial pressure, with or without simultaneous bloodpressure measurement, in freely moving individuals that are notbed-ridden. Therefore the apparatus is small and may be driven by arechargeable battery.

[0037] Another object of the present invention is to provide anapparatus for recording and storing a large number of intracranialpressure recordings, that is pressures sampled at least 10 times asecond, and more preferably 100 to at least 150 times a second, for atleast 24-48 hours. Preferably the frequency by which pressure is sampledmay be selected by the physician, ranging from about 10 to at least 150Hz. The data may be transferred via the serial port to a personalcomputer or network connection for further analysis. During monitoringof single pulse pressure waves, a frequency of 100 Hz will be acceptablefor monitoring single waves, with parameters related to the first peak(P1).

[0038] Another object of the present invention is to provide anapparatus that may record signals indicative of the intracranialpressure or blood pressure from various sources of signals, that isinvasive implanted microtransducers and non-invasive devices usingacoustic or ultrasonic signals, or other signals recorded bynon-invasive devices. Thus, the algorithm for analysis of pressures maybe used whether pressure signals are derived from invasive ornon-invasive devices.

[0039] Another object with the invention is to provide a technicalsolution for monitoring intracranial pressures without being dependenton the zero level (i.e. calibration against the atmospheric pressure).This is particularly important for pressure sampling by means ofnon-invasive sensors. An object of the invention is to provide asolution for analysis and presentation of continuous intracranialpressure recordings obtained by non-invasive sensors.

[0040] Another object of the present invention is to provide anapparatus that may serve as an interface between the patient and amonitor/network station allowing online monitoring of the intracranialpressure.

[0041] Another object of the present invention is to provide a newmethod of analyzing pressure samples such as intracranial pressure,blood pressure or cerebral perfusion pressure, including quantitativepresentations of the various pressure curves. The different pressuresmay be monitored simultaneously.

[0042] Yet another object of the present invention is to providesoftware for the quantitative analysis and presentation of continuouspressure recordings representing e.g. intracranial pressure, bloodpressure and cerebral perfusion pressure. The software has severaloptions for quantitative description of the data, including calculationof a matrix of pressure elevations of different levels and durations, ora matrix of pressure changes of different levels and durations, or amatrix of numbers of single pulse pressure wave parameters with selectedcharacteristics.

[0043] The main objectives of the invention are related to intracranialpressure and blood pressure, but this is not a limitation on the scopeof the invention. The invention can also be utilized in connection withpressure sensors measuring pressure in other body cavities (such as thecerebrospinal fluid cavities).

[0044] In light of the above-mentioned objectives, a method has beendeveloped for measuring and analyzing pressure in a patient. Accordingto this method one or more pressure sensors are applied to a patient andthe pressure signals from the sensors are sampled at selected intervals.The sampled signals are converted to digital form and stored along witha time reference that makes it possible to evaluate the change ofpressure over time. The time reference may be stored as part of thedigital value, or it may be associated with the memory position, ormemory address, at which the pressure value is stored. The stored samplevalues are then, according to this embodiment of the invention, analyzedin order to generate a presentation of at least one of the following:number of pressure elevations with any selected combination of level andduration; number of pressure changes with any selected combination oflevel difference and duration of change; and number of pulse pressurewaves with preselected characteristics regarding minimum, maximum,amplitude, latency and rise time. The method allows for various samplingrates and duration of measuring periods. Assessment of single pulsepressure waves preferentially requires a sampling rate of 100 Hz orabove. As an alternative to numbers, percentages may be computed. Anypoint of the single waves may be calculated, and different parameters ofthe waves may be computed. There is a fundamental difference betweencomputation of number of pressure elevations with any selectedcombination of level and duration and number of pulse pressure waveswith preselected characteristics regarding minimum, maximum, amplitude,latency and rise time. The first method computes pressures relative to azero level (i.e. atmospheric pressure), whereas the latter methodcomputes relative differences in pressures and time and therefore isindependent on the zero level.

[0045] One object of the invention is to provide a system for handlingsingle pulse pressure waves in a way that pressures from a singlesubject may be superimposed on the pressure-volume (elastance) curveproviding information about the elastance. This solution provides one ofseveral strategies of early detection of decompensation of pressures,before the conventional methods.

[0046] One object of the present invention is then to provide a systemfor quantitative and accurate comparisons of pressure recordings/curveswhen assessing pressure in a body cavity or blood pressure. Comparisonsmay be made between different continuous pressure curves that includedifferent recording periods, different heart rates, as well as differentzero levels. Comparisons of continuous pressure recordings may be madeboth between individuals and within individuals (that is before andafter treatment or comparisons of pressure recordings at different timeintervals). This system includes a new algorithm integrated in computersoftware. The algorithm includes quantitative approaches for analysis ofthe pressure recordings and strategies to present the recordings. Thesystem may be integrated in commercially available pressure transducerdevices, in computer servers or in medical device computers or in theportable apparatus for pressure monitoring described here.

[0047] The technical solution of comparing various continuous pressurecurves involves standardisation procedures. The numbers/percentagesduring a given recording period may be standardized tonumbers/percentages during a standardized recording period (e.g. one or10 hours) and a standardized heart rate. For different individuals thequantitative data for a given recording period may be standardised to aselected recording period (for example numbers/percentages during oneminute, one hour or 10 hours recording period), as well as standardisedto a selected heart rate (for example heart rate of 60 each minute).Thereby, continuous pressure recordings for different individuals may becompared. This strategy may provide the opportunity for development ofreference curves, on the basis of recordings in several individuals.Comparisons of pressure curves for individual cases also becomepossible. During real time and on-line pressure monitoring, changes inpressure trends may be explored. For example, numbers of pressurecharacteristics during one hour of pressure recording may be compared atdifferent time intervals.

[0048] As compared to the traditional monitoring of mean intracranialpressure, assessment of parameters of single waves may provide earlywarning of changes in brain compliance, allowing early intervention toreduce pressure.

[0049] According to one aspect of the invention, an apparatus forperforming the information gathering according to the method has beendeveloped. The apparatus is small enough to be carried by a patient, sothe patient will be free to move about during the measuring period. Theapparatus comprises means for connecting to one or more sensors, aconverter for producing the digital measuring values, a processorcontrolling the sampling of the measuring signals and storing thedigital values in a data memory. The apparatus also comprises aconnector for connecting the apparatus to external computing means inorder to upload values stored in the data memory or to deliver samplingvalues in real time to said external computing means.

[0050] Another aspect of the invention concerns a system for performingthe analysis according to the method. The system may be in the form of asuitably programmed computer, or dedicated equipment particularlydesigned for performing this analysis. The system includes acommunication interface for receiving a set of digital pressure samplevalues, a memory for storing these values, and a processor forperforming the analysis described above. The system further includes avideo interface that is controlled by the processor and that is capableof generating a visual presentation of the result of any analysisperformed by the processor. The visual presentation will be presented ona display. The system also comprises input means for allowing a user tochange the parameters of the performed analysis. This implies that thesystem may be integrated in different computer servers, medical devicecomputers or vital sign monitors. Therefore, the apparatus describedhere represents no limitation by which the invention may be applied.

[0051] The output computed by the software may be presented in a numberof ways, including matrix of numbers, graphical presentations, andcomparisons of pressures in an individual against a reference materialor against previous recordings of the individual.

[0052] Finally the invention includes a computer program product forcontrolling a computer performing the analysis described above. Thecomputer program may be installed on a computer or carried on a carriersuch as a CD ROM a magnetic storage device, a propagated signal carryinginformation, or in any other manner known in the art.

[0053] The particular features of the invention are described in theattached independent claims, while the dependent claims describeadvantageous embodiments and alternatives.

BRIEF DESCRIPTION OF THE DRAWINGS

[0054]FIG. 1 is a block diagram of the various components of a systemaccording to the invention.

[0055]FIG. 2 is a graphical user interface used for presentingpressure-sampling results.

[0056]FIG. 3 is a graphical user interface for presenting andcontrolling the analysis of a pressure curve.

[0057]FIG. 4 shows a part of the graphical user interface of FIG. 3 fordifferent levels and duration's.

[0058]FIG. 5 is a graphical user interface for presentingpressure-sampling results.

[0059]FIG. 6 is a presentation of comparisons of pressure curves withinan individual.

[0060]FIG. 7 is a presentation of the parameters measured duringanalysis of single pulse pressure waves.

[0061]FIG. 8 is parts of graphical user interfaces for presentation ofsingle pulse pressure waves.

[0062]FIG. 9 is graphical user interfaces for presentation of pressurerecordings and parameters of single pulse pressure waves during aninfusion test.

[0063]FIG. 10 is a presentation of comparisons of parameters ofdifferent types of single pulse pressure waves.

[0064]FIG. 11 is a block diagram of different applications of theinvention.

[0065]FIG. 12 presents for three different patients the pressure curvesand the accompanying histograms of single wave distribution.

[0066]FIG. 13 presents the pressure curves and the accompanyinghistograms of single wave distribution for simultaneous intracranialpressure recordings via both intraparenchymatous and epidural sensors.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0067]FIG. 1 illustrates in a block diagram a system for measuringpressure in a body cavity of a patient. The main components of thissystem includes a pressure sensor 16, a pressure transducer 2, aportable apparatus for measuring and storing pressure values 1, and anetwork station such as a personal computer 6 for receiving andprocessing registered pressure values. The apparatus 1 is a digitalsystem with a central processing unit 8 for sampling and storingpressure measurements in a patient, such as intracranial pressure, bloodpressure or pressure in other body cavities or blood pressure. In thefollowing example an embodiment for measuring intracranial pressure willbe described, but it must be understood that this is not a limitation onthe scope of the invention.

[0068] As a result of its compact construction and lightweight, apatient can easily carry the apparatus 1. The apparatus 1 may befastened to the belt of the patient or kept in a carry pouch withstraps. Alternatively, the apparatus 1 may be used as an interface forconnecting the network station or personal computer 6 to the pressuresensor 2. This allows real time online monitoring of pressure so thatthe pressure curves may be displayed on a display. The differentapplications of the apparatus 1 as well as modifications in theconstruction of the apparatus 1 are further illustrated in FIG. 11.

[0069] In an embodiment of the invention, the pressure sensor 16 isimplanted within the body cavity within which pressure is to bemeasured, such as the skull of the patient. The sensor 16 is connectedto a pressure transducer 2, which in turn is connected to the apparatus1 via a connector 4. People known in the art would know that sensor andtransducer are not identical in the sense that the sensor itself detectsvarious types of physical forces, whereas the transducer converts thesignal from the sensor to either a voltage or a current signal. There isno limitation of the type of pressure sensors that may be connected tothe apparatus 1. Various types of pressure sensors 16 and transducers 2are commercially available, including transducers for non-invasivepressure assessment, using acoustic, or other types of signals. Othersensors 16 may detect pressure within a fluid space such as thecerebrospinal fluid space. Independent of the type of signals, thesignals are converted from analogue to digital form in an analogue todigital converter 7.

[0070] Most commercially available sensors 16 give an analogue signal onthe basis of a mechanical action on the sensor. Within the pressuretransducer 2 the signals from the sensor is converted to a signal thatmay either be a voltage or current signal. The pressure transducer 2then produces a continuous voltage or current signal. The voltage orcurrent signals from the transducer are further processed within thesignal conditioner 5. The analogue signals are converted to digitalsignals within the analogue to digital converter 7. Certainly variousmodifications are possible. When data are collected from for example avital signs monitor both the pressure transducer 2 and the analogue todigital converter 7 may be built into the vital signs monitor. Thedigital signals are handled according to the invention.

[0071] The apparatus 1 may be constructed in a number of ways. Theembodiment described below is based on a unit with a central processingunit 8 operating in accordance with instructions stored in memory 9 andcommunicating with the various parts of the apparatus over a common databus 14. However, a number of variations are possible. Instead of using acentral processing unit 8 and instructions stored in memory 9, thefunctionality of the apparatus 1 could be constructed directly inhardware, e.g. as ASICs. The apparatus represents no limitation for theuse of the system for the analysis and presentation of pressuresdescribed here.

[0072] The main components of the apparatus 1 are then the analog todigital converter 7, which converts the received analog measuringsignals to digital, the data memory 9, which receives the digitizedvalues from the analog to digital converter 7 and stores them. Aninput/output interface 15 allows data stored in the memory 9 to betransferred to the network station or personal computer 6 forprocessing. The apparatus preferably includes a galvanic element 3protecting the patient from the electric circuitry of the apparatus, asignal conditioner 5 either to the input or the output of the analog todigital converter 7, an input control 10 for controlling operation andadjusting settings of the apparatus, a display unit 12, and an alarmunit 13. Input control 10, display 12 and alarm unit 13 are connected toand in communication with the central processing unit 8 and/or otherparts of the apparatus such as ASICs, display drivers, and power sensors(not shown).

[0073] After being received by the apparatus over a connector 4 to whichthe pressure transducer 2 is connected, analog measuring signals aresent to a signal conditioner 5. Preferably a galvanic element 3 ispositioned between the interface 4 and the signal conditioner 5,representing a security element preventing electrical energy from beingsent retrograde to the patient. Signal processing in the conditioner 5modifies the signal-to-noise ratio. This is required since a high degreeof noise can be expected for instance during walking. The signalconditioner 5 may be an analog filter. Alternatively, the signalconditioner 5 may be a digital filter operating under control of thecentral processing unit 8. The signal conditioner 5 will then bepositioned following the conversion of the sampled signal from analog todigital.

[0074] Besides, the software computes the number of artifacts during arecording period, and the artifact ratio. The program includes an optionfor excluding recordings when the artifact ratio is above a selectedlevel.

[0075] After the signal conditioner 5 has processed the analog signals,the analog signals are converted to digital signals within an analog todigital converter 7. The central processing unit 8 controls theoperation of the various elements of the apparatus 1. The centralprocessor is in communication with the analog to digital converter 7,and is capable of reading out samples of the digitally convertedpressure measurements and storing them in a data memory 9. The datamemory 9 may be in the form of electronic circuits such as RAM, or someform of magnetic storage, such as a disc, or any other convenient formof data memory known in the art.

[0076] As has already been mentioned, the apparatus 1 is here describedas receiving signals indicative of the intracranial pressure fromsensors 16 implanted within the skull. However, the apparatus may alsoincorporate a signal conditioner 5 for processing signals fromnon-invasive devices such as acoustic, ultrasonic or Doppler devices.Whether the entire apparatus 1 must be constructed with a signalconditioner 5 for a specific purpose or whether the same signalconditioner 5 allows for different uses, with or without re-programming,is dependent on implementation and specific needs. If the apparatus 1 isintended to work with various sensors 16 with various levels ofsensitivity, the signal conditioner should be adjustable in a mannerthat allows operation with the desired sensors and to adapt the outputrange to the various sensors to the input range of the analog to digitalconverter 7. In this case the signal conditioner 5 must obviously beconnected between the input of the apparatus 1 and the analog to digitalconverter 7.

[0077] The apparatus 1 is programmable including an input control 10,with a simple key board for entering a few commands. The input control10 has a calibration function that allows calibration of the pressuresensor 16 against the atmospheric pressure, before the sensor 2 isimplanted within the skull of the patient. Thereby the intracranialpressure monitored actually is the difference between the atmosphericpressure and the pressure within the skull of the patient. It should benoted, however, that this invention also describes a method forrecording and analysis of relative continuous pressure recordings thatare not related to the atmospheric pressure, and are independent of azero level. The input control 10 also contains a function for selectingthe interval of pressure recordings. The pressures may be recorded withvariable sampling frequency, e.g. from about 1-10 Hz up to at least 150Hz (most preferably between 100 and 200 Hz). When single pulse pressurewaves are monitored, the sampling frequency preferentially is 100 Hz orabove. The minimum memory space should then allow storing of recordingsat least 150 times a second for at least 48 hrs (26 920 000 recordings).The input control 10 preferably also has a function for adjusting thereal time clock, since each pressure sample should include a timereference indicating when the sample was made.

[0078] Functions on the input control 10 for the physician preferablyincludes the following: On/Off, calibration, protocol (frequency rate ofpressure sampling), start and clock adjustments. Functions for thepatient or the nurse may include Day/Night and Events.

[0079] Via a connector 11, data may be transferred to the personalcomputer 6 for analysis. The connector 11 may be a serial port, and theapparatus will preferably comprise an input/output interface 15converting the internal signal format for the apparatus 1 to a formatfor communication over said connector 11.

[0080] A display 12 shows on-line the digital pressure signals as wellas the real-time time. The display 12 is preferably controlled by thecentral processing unit 8.

[0081] An internal battery (not shown) powers the apparatus 1 thatpreferably is rechargeable, but with input for external power supply(not shown).

[0082] In a preferred embodiment, the apparatus 1 has an alarm functionthat indicates shortage of memory capacity or reduced battery capacity.This alarm may be displayed visually on the display 12, but may alsoinclude a unit 13 emitting an audible alarm signal.

[0083] In addition to the battery that powers the apparatus 1 while inuse, the apparatus 1 may include an additional battery that serves tomaintain data in the volatile part of the memory 9 when the main batteryruns low or is removed. Alternatively, or in addition, the alarmfunction described above may, upon detecting low power status of themain battery, trigger a routine that transfers any data in the volatilepart of the memory to a non-volatile part of the memory. The volatilepart of the memory may be the working RAM of the apparatus 1, while thenon-volatile part of the memory may be any combination of ROM, EEPROM, amagnetic storage medium or any other such memory known in the art.People skilled in the art will, however, realize that otherconfigurations of memory are possible within the scope and spirit of theinvention.

[0084] As mentioned before, the apparatus 1 may be connected to apersonal computer 6 via the serial port 11. Alternatively the apparatus1 may be connected to another digital computer-based monitoring system 6such as a network station. This gives the opportunity for on-line andreal time monitoring of the pressure with real time graphic presentationof the recordings. In this situation the apparatus 1 functions as aninterface for a stationary personal computer or flat screen. Differentapplications are illustrated in FIG. 11.

[0085] The apparatus 1 is preferably controlled by software that isstored in a non-volatile part of the memory 9, and that controls theoperation of the central processor 8. The various units of the apparatusare shown as communicating over a common data bus 14, but it should benoted that the various components may be interconnected in other ways.

[0086] The apparatus 1 has been described above with only one channelfor receiving pressure signals from one pressure sensor. The apparatusmay, as mentioned before, include one or more additional channels forreceiving signals from additional pressure sensors. According to apreferred embodiment of the invention the apparatus comprises two inputchannels, allowing the simultaneous recording of e.g. intracranialpressure and blood pressure. An embodiment with more than one inputchannel will comprise additional connectors 4 and galvanic elements 3.The signal conditioner 5 and analog-to-digital converter 7 may besimilarly duplicated, or one signal conditioner 5 and/or oneanalog-to-digital converter 7 may operate the several pressure signalchannels in a multiplexed manner, controlled by the central processingunit 8. If the apparatus comprises several channels, the capacity of thedata memory 9 must be increased accordingly.

[0087] The invention also relates to a method for measuring andanalyzing pressure in a patient. This method will now be described.

[0088] First a signal from a pressure sensor 16 and transducer 2representative of pressure in a body cavity is received and sampled atselected intervals. This signal is converted to digital form 7 andstored along with a time reference representative of the time at whichthe sample was made 9. The time reference does not have to be a timereference value stored for every sample. Since the sample rate will beknown, it will be sufficient to store an actual time reference for thestart of the measuring period. The time reference for the individualsamples will then be given by their relative address in memory.

[0089] The stored sample values may then be analyzed in order togenerate a presentation regarding a time period of at least one of thefollowing:

[0090] number of pressure elevations with any selected combination oflevel and duration,

[0091] number of pressure changes with any selected combination of leveldifference and duration of change,

[0092] number of single pulse pressure waves with pre-selectedcharacteristics such as minimum, maximum, amplitude, latency and risetime.

[0093] This type of analysis may be performed either on-line oroff-line. During on-line analysis, analysis is performed repeatedly andpresented repeatedly during real-time on-line monitoring. This allowsfor comparisons of pressure characteristics at repeated intervals.Off-line analysis is performed after the recording period has beenended.

[0094] In order to analyze number of pressure elevations with anyselected combination of level and duration occurring in a time period,the stored samples are simply analyzed in order to determine for howlong the measured pressure has remained within a certain pressureinterval. According to a preferred embodiment of the invention, the userperforming the analysis will be able to set the pressure intervalsdefining the various levels and duration of pressure elevations manuallyand perform the analysis repeatedly with different values for theseparameters. Level may be measured on a linear scale e.g. with intervalsof 5 mmHg, while the time scale intervals should preferably increasewith time, e.g. each interval being twice as long as the previousshorter interval.

[0095] An analysis of number of pressure changes with any selectedcombination of level difference and duration of change would involve ananalysis of the stored samples in order to determine the size of apressure change and the time over which the change takes place.

[0096] An analysis of single pulse pressure waves will take intoconsideration not only elevations that remain within a certain timeinterval, but the transition of a wave from minimum to maximum and backto a new minimum or vice versa. Pre-selected characteristics identifyinga pressure wave of interest may be the duration of the single pulse wavefrom minimum (maximum) back to minimum (maximum) combined either withminimum value, maximum value or amplitude of the single wave. Anotherpre-selected characteristic may be the rise time of the single wave.

[0097] The pressure sensor 16 may be applied by implanting the sensor ina body cavity of the patient, but it may also be applied by anon-invasive technique with a sensor using acoustic measuring signals,ultrasonic or Doppler, or even a pressure sensor for measuring bloodpressure. In general, a problem with non-invasive sensors recordingintracranial pressure, is the lack of a zero level since intracranialpressure is calibrated against atmospheric pressure. The presentinvention solves this problem by computing the relative differences inpressure during single pressure wave analysis. Thereby the need for azero level is excluded.

[0098] As a result of the small size of the apparatus, the sampling andstoring of pressure signals may be made while the patient is free tomove about. The analysis is preferably performed by transferring theaggregated data to a computer 6 for analysis and graphical presentation.The presentation generated as part of this analysis may be in the formof absolute numbers, percentages or numbers per time unit.

[0099] According to a preferred embodiment, the sampling rate is atleast 10 Hz, and the measurements may be taken over a period of at least24 hours. Even more preferably, the measurements may be performed with asampling rate of 100 Hz, or at least 150 Hz, and taken over a period ofat least 48 hours. According to the preferred embodiment of theapparatus the physician can set the sampling rate through the inputcontrol 10.

[0100] The computer 6 performing the analysis of the aggregated pressuredata may be a regular personal computer or a dedicated unit forperforming the analysis and generating presentations of the results. Thecomputer embodies a system for analysis of recorded pressure data inaccordance with the invention.

[0101] The computer is not shown in detail. It preferably includes astandard communication interface for receiving a set of digital pressuresample values from the apparatus described above, as well as datamemory, such as a hard drive, for storing the received sample values andprocessing means, such as a microprocessor, with access to said datamemory, and capable of analyzing said sample values in order todetermine at least one of the following:—number of pressure elevationswith any selected combination of level and duration—number of pressurechanges with any selected combination of level difference and durationof change,—number of single pulse pressure waves with preselectedcharacteristics regarding minimum, maximum, amplitude, latency and risetime. The computer further includes a video interface in communicationwith said processing means and capable of, in combination with theprocessor means, generating a visual presentation of the result of anyanalysis performed on the pressure sample values together with agraphical user interface. The video interface may be a graphics cardconnected to a display for displaying the generated visual presentation.The computer will also include input means allowing a user of the systemto enter and change parameters on which said analysis should be based.These input means will normally include a keyboard and e.g. a mouse, andthe user will be assisted by a graphical user interface presented on thedisplay.

[0102] The parameters on which the analysis should be based may includeat least some of the following: pressure intervals defining a number ofpressure elevations, pressure change intervals defining a number ofpressure change step sizes, time intervals defining a number ofdurations, pressure wave characteristics including minimum, maximum,amplitude and latency, selection of type of analysis, and selection ofpresentation of numbers as absolute numbers, percentages or numbers pertime unit.

[0103] The operation of the computer 6 will preferably be controlled bycomputer program instructions stored in the computer 6 and making thecomputer capable of performing the analysis. The program will preferablybe able to perform the analysis based on default values in the absenceof parameters input by a user. Such a computer program may be stored ona computer readable medium such as a magnetic disc, a CD ROM or someother storage means, or it may be available as a carrier signaltransmitted over a computer network such as the Internet.

[0104]FIG. 2 illustrates the graphical user interface of the computersoftware used for presenting the results of the sampling describedabove. The software processes the digital pressure signals. Before thecontinuous pressure recordings are presented in the graphical userinterface as shown in FIG. 2, the pressure signals are sampled andaveraged. With regard to FIG. 2, the sample update rate was in the range30 to 100 Hz and the update rate (average interval) was in the range 1to 5 seconds. The update rates may vary between 1-10 Hz for lowfrequency sampling. Modern vital signs monitors may offer a computerinterface producing this type of averaging. Various modules of thesoftware generate output or can be invoked through this interface. Theintracranial pressure curve 34 may be presented in various windows. TheX-axis shows the time of registration 20, that is real time ofintracranial pressure sampling (presented as hours: minutes: seconds).The Y-axis 21 shows the absolute intracranial pressure recordings(presented as mmHg). During the recordings, it is possible to markevents (e.g. sleep, walking, sitting) and these may be presented assymbols 22 along the X-axis above the pressure graph. There arefunctions 33 for selecting the recording periods, for instance selectingparts of the intracranial pressure curve during sleep, walking, sittingetc. There are functions for selecting different window sizes 23 bothvertically and horizontally. The curve 34 presented in the window inFIG. 2 represents about 21 hours recording time (that is actualrecording time). A special function 24 allows simple statisticalanalysis of the data presented in the window (with calculations of mean,standard deviation, median, ranges and time of recording). Anotherfunction 25 transfers to a software module that performs quantitativeanalysis of a single intracranial pressure curve in accordance with theinvention. The results of this analysis are described below withreference to FIGS. 3-6. Another function 26 allows export ofintracranial pressure data from a selected window to files with aselected text format such as ASCII, that can be utilized by e.g.spreadsheet or word processing applications. The intracranial pressurecurve may be smoothened by another function 27. Another function allowsprinting of the intracranial pressure curve 28. The software alsoincludes a function for patient identification 29 also containing somedata of the patient (such as tentative diagnosis and cause ofexamination). In addition, there are start 31 and stop 32 buttons forcontrolling the sampling process. If the apparatus has collectedpressure samples from several pressure transducers 2, e.g. intracranialand blood pressure, these may be simultaneously analyzed. The functionsare linked up to the pressure recordings displayed in the window. Anytype of pressure may be presented in this way.

[0105] The size of the window, that is the observation time may bechanged to reveal the single pulse waves. Each single pulse wave isbuilt up from a blood pressure wave. Comparable to the heart rate,during one minute of recording often about 50-70 single pulse waves maybe recorded. There is, however, a large variation in heart rate bothbetween and within individuals, accordingly there is a variation in thenumbers of single pulse intracranial or blood pressure waves during oneminute recording.

[0106] The graphical interface in FIG. 2 represents one example ofpresenting/displaying the various functions. Various modifications arepossible. Simultaneous presentations of the continuous pressurerecording curves of different pressures (e.g. intracranial pressure,blood pressure, cerebral perfusion pressure) may be presented in thesame window. The continuous recordings are presented real time so thatthe different types of pressures may be compared. Modifications in thegraphical interface may be performed whether the pressure monitoring isintended for on-line or off-line monitoring. During on-line monitoring,statistical analysis may be computed repeatedly, to allow comparisonsbetween different time intervals. The real-time continuous pressurecurve may be presented in one window, the absolute pressure parameters(such as mean pressure, standard deviation, and ranges) in anotherwindow and single waves in still another window.

[0107] The functions referred to above and the software modules thatperform them will not be described in detail as they are well known inthe art and do not constitute a part of the invention as such.

[0108] Reference is now made to FIG. 3 which shows the graphical userinterface of the software module for analysis of the intracranial orblood pressure curve, or other pressures in human body cavities. Theselected window of the intracranial pressure curve 34 is presented as achart or matrix 35 of quantities of different types, derived through theinvented method of analysis. Any size of the recording period 33represented by the window may be selected for the quantitative analysis.A similar user interface is used independent on the type of pressuremeasured.

[0109] The mathematical functions may be implemented in the software byvarious routes. One implementation is shortly described. The data neededfor analysis of pressure elevations of different levels and durationsinclude the pressure recordings and the corresponding time recordings.Two variables are selected, namely the threshold levels (pressuresexpressed in mmHg) and the width (time expressed in seconds). A searchis made for both peaks (positive-going bumps) and valleys(negative-going bumps), and the exact levels of peaks and valleys areidentified. Peaks with heights lower than the threshold or valleys withtroughs higher than the thresholds are ignored. For a threshold valueless or equal to zero a valley search is performed. For threshold valuesgreater than zero a search for peaks is performed. The peak/valleysanalysis is performed for every width/threshold combination in thematrix. In short, the procedure is as follows. The part of the pressurecurve 34 that is to be examined is selected 33, the data is visualisedin the user interface. A suitable width/threshold matrix is selected,specifying the width/threshold combinations. The units used are time inseconds (width) 37, and pressure in mmHg (threshold) 36, respectively.The software records the numbers of samples that fit a givenwidth/threshold combination. The output from the analysis is a matrixcontaining the numbers of all the different width and thresholdcombinations. An example of such a matrix 35 is given in FIG. 3. Asshown in the matrix 35, the width/threshold combination 20 seconds/25mmHg (that is ICP elevations of 25 mmHg lasting 20 seconds) occurred63.00 times during the actual recording time of 21.10 hours 45. In thismatrix the numbers were not standardised to a selected recording period42. The pressure elevations are relative to the zero level thatcorresponds to the atmospheric pressure.

[0110] By clicking a first button 38, the user can select a presentationof the data as a chart of numbers of intracranial pressure elevationswith various combinations of level 36 and duration 37. The intracranialpressure levels and durations may be selected in each case. According toa preferred embodiment, intracranial pressure is expressed as mmHg andduration as seconds and minutes. Also blood pressure may be expressed asmmHg. Independent of the type of pressure measured the pressures may bepresented in the same way.

[0111] A second button 39 allows the user to select presentation of thedata as a chart of numbers of intracranial pressure intracranialpressure changes of different levels 36 and duration 37. The changes maybe differences between two recordings or differences between a recordingcompared to a given or selected value (e.g. mean pressure).

[0112] By clicking a third button 40, the user selects presentation ofthe data as numbers of single pulse pressure waves with pre-selectedcharacteristics. The user accesses an input dialog box for enteringthese characteristics by clicking a fourth button 41. Each single pulsepressure wave is identified by minimum, maximum, amplitude, latency andrise time. Further details about analysis and presentation of theparameters of single pulse pressure waves are given in FIGS. 7-10.

[0113] The presentation of the results of the analysis in chart 35 maybe toggled between absolute numerical quantities and percentages ofrecording time by clicking one of two buttons 44.

[0114] The numbers may be standardized by presenting the data as numbersper time unit 42. The time unit (e.g.) may be selected in eachindividual case. The data presented in FIG. 3 was based on a recordingtime of 21.1 hrs (actual recording time 45), and the recordings were notstandardized in this case (represented by zero in standardization inputbox 42). It should be noted that standardization may be performed tovarious time units, such as each one minute, one hour or even 10 hours.Since the calculation of single pulse pressure waves automatically alsogives the heart rate it is possible to standardize the numbers accordingto a given heart rate (further details given in FIG. 7). For example,the numbers may be standardized to a given heart rate of 60/min.

[0115] The opportunity to standardize the numbers presented in thematrix is important for comparisons of pressure recordings, eitherwithin or between individuals. Thus, two matrixes 35 may be compared,for example at two different times in one individual. For example, itmay be possible to calculate the numbers of single waves with certaincharacteristics (defined by parameters such as rise time and amplitude)during a recording time.

[0116] During on-line presentation the matrix 35 may be comparedrepeatedly. The whole matrix 35 may not need to be presented but onlycertain width/threshold combinations. Differences between certaincombinations at different time intervals may be revealed. For example,the numbers or percentages of intracranial pressures of 15, 20 and 25mmHg lasting 5 minutes during 1 hour recording period may be computedand presented each hour during on-line presentation. Normalization ofdata to a standardized recording time 42 and heart rate allows foraccurate comparisons between different time intervals for individualcases, as well as comparisons between individuals.

[0117] For example, for blood pressure, comparisons of pressure curvesmay be performed before and after treatment with medications in anindividual. Alternatively, pressure recordings from an individual may becompared against a normal material. A normal material may be constructedon the basis of the recordings from a large group of individuals.

[0118] The method for performing these analyses is described above, andthe various buttons described above invokes software modules forperforming the various steps of this method.

[0119] Again, a special function 43 allows the analyzed data to be savedas text files with a selected text format such as ASCII, or other filescompatible with applications for mathematical and/or statisticalhandling of the data or for generating presentations.

[0120]FIG. 4 shows part of the graphical user interface of FIG. 3 with adifferent set of parameters. In particular, the various time intervalsof duration 37 have been changed, and the matrix 35 shows numbers ofelevations normalized as number of occurrences per time unit 42. In thiscase the numbers are derived from a standardized recording time of 10hours 42, with the actual recording period 9.01 hrs 45.

[0121] The results shown in FIG. 3 are the results of an analysis ofnumber of pressure elevations with selected combinations of level andduration. As indicated in FIG. 4, the stored samples have been analyzedin order to determine for how long the measured pressure level 36 hasremained within a certain pressure interval, represented as −10, −5 0,5, 10, 15, 20, 25, 30, 35, 40 and 45 mmHg relative to atmosphericpressure, for certain periods of time 37. The various periods of time 37are selected as 30, 60, 300, 600, 1200 and 2400 seconds, respectively.In FIG. 4, the results have been normalized to numbers during a 10 hoursrecording period 42. Among the results in the result matrix 35 it can beseen that intracranial pressure elevations of 45 mmHg with a duration of30 seconds have occurred 8.88 times when normalized to a 10 hourmeasuring period. Similarly, pressure elevations of 30 mmHg with aduration of 600 seconds have occurred 2.22 times when normalized to a 10hrs recording period. In FIG. 3, where the results are not normalized,all the results are integers.

[0122] During the standardisation procedure, the numbers or percentagesare adjusted to a given factor. The normalised time may be chosen ineach individual. An example is given. If the actual recording time is 6hours, a standardisation to 10 hours recording time implies that allnumbers or percentages of pressure elevations are multiplied with afactor equal to 10/6 (that is 1.66666).

[0123] The following example is intended to illustrate various aspectsof the present invention regarding related measurements of pressurewaves described in FIGS. 2-4, but is not intended to limit the scopethereof.

EXAMPLE 1

[0124] Continuous intracranial pressure monitoring was performed in agirl aged 2 years and 11 months because of suspected shunt failure. Inthis girl an extracranial shunt was previously placed because ofhydrocephalus. Shunt failure was suspected because of headache, lethargyand irritability. In fact, increased, reduced or normal intracranialpressures may cause these symptoms. The results of intracranial pressuremonitoring during sleep in this girl were as follows: Mean intracranialpressure 14.4 mmHg, range 0.1-67.3 mmHg, std 5.7 mmHg. The duration ofintracranial pressure monitoring was 544 minutes. A mean pressure of14.4 mmHg is by most physicians considered as borderline whereas apressure above 15 mmHg is considered as abnormal. Therefore, noindication for surgery (shunt revision) was found on the basis of theintracranial pressure monitoring. The girl was not treated whichresulted in lasting symptoms of headache and lethargy for more than 2years. A retrospective analysis of the intracranial pressure curve wasperformed by means of the method according to the invention. FIG. 4shows a matrix of intracranial pressure elevations of different levelsand durations that was calculated, clearly demonstrating a high numberof abnormal intracranial pressure elevations, for instance a high numberof intracranial elevations of 25 mmHg or above. During a standardizedrecording time of 10 hours, intracranial pressure elevations of 25 mmHglasting 300 seconds occurred 6.66 times. Such elevations generally areconsidered as abnormal. This case serves as an example of anintracranial pressure curve that was misinterpreted because the curvewas interpreted on the basis of classical criteria. Mean intracranialpressure was within acceptable values. Application of the presentsoftware added significant new information that would have changed thedecision making in this patient.

[0125]FIG. 5 shows the same part of the graphical user interface as FIG.4, but in this case the analysis is an analysis of number of pressurewith selected combinations of level difference 30 and duration of change37. The stored samples have been analyzed in order to determine thenumber of pressure changes of certain sizes 30, represented as −20, −15,0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 mmHg relatively, and the duration 37over which these changes take place, given as 10, 15, 20, 25, 30, 35,40, 45 and 50 seconds. Among the results given in the result matrix 35it can be seen that a pressure change of 2 mmHg that takes place over a15 seconds has occurred on average 1.14 times per 10 hour period.Changes of 0 mmHg represent periods of time over which the pressure hasremained constant. Also in this matrix the numbers have beenstandardized to numbers during a 10 hours recording period. Thestandardization procedure gives the opportunity to compare pressurecurves, either within individuals at different time intervals or betweenindividuals.

[0126] Different strategies may be used to calculate numbers of changesover a given period of time. One implementation is described. If thesignal represented by X has n samples, it is possible to find n−1changes with a given interval (l). The changes represented by J_(i) willbe equal to the sequence of elements represented by the sample number(X₁, X_(i+1)) for i=0, 1, 2, . . . n−1. The changes itself J_(i) will berepresented by X_(i+1)−X_(i). After this procedure, the numbers from thelatter analysis is then inspected for a given change. If this number isfound a counter is increased with one. If a number of intervals andchanges are put in a matrix where the rows represent time and thecolumns represented intervals. This procedure produces aneasy-to-understand presentation of various changes of various intervals.If the matrix has a number of A rows, and B columns. The sequence J_(i)mentioned above has be done A×B times.

[0127] The procedure of comparing pressure curves 34 is furtherillustrated in FIG. 6. The pressure curves before (left) and after(right) surgery are shown, and below the matrix 35 of numbers ofpressure elevations. The numbers are standardized to a 10 hrs recordingperiod 42. More details are given in Example 2 below. This example isintended to illustrate various aspects of the invention described inFIGS. 2-6, but is not intended to limit the scope thereof.

EXAMPLE 2

[0128] Continuous intracranial pressure monitoring was performed in a 3years and 10 months old boy due to suspected premature closure of thecranial sutures. The boy had symptoms of increased intracranialpressure. During sleep the data of the intracranial pressure curve wereas follows: Mean intracranial pressure 15.4 mmHg, range 0-57.1 mmHg, std6.0 mmHg, and time of pressure recording 480 min (8.0 hrs). On the basisof the results of intracranial pressure monitoring, surgery wasperformed. A cranial expansion procedure that is a rather majorprocedure was performed to increase the cranial volume and therebyreduce intracranial pressure. However, after surgery the patient stillhad symptoms of intracranial hypertension. Therefore it was decided torepeat the intracranial pressure monitoring, that was undertaken sixmonths after surgery. The data for this monitoring during sleep were asfollows: Mean intracranial pressure 15.2 mmHg, range 5.5-39.4 mmHg, std3.9 mmHg, and time of intracranial pressure recording 591 min (9.85hrs). This new intracranial pressure monitoring was inconclusive becausemean intracranial pressure was unchanged after surgery. In retrospect,the monitoring of intracranial pressure was without purpose since noconclusions could be drawn on the basis of the pressure recordings.Though the pressure was unchanged after surgery, it was decided not toperform a new operation though the results of intracranial pressuremonitoring did not document any reduction of intracranial pressure aftercranial expansion surgery. A “wait and see” policy was chosen on thebasis of intracranial pressure monitoring. However, when the methodaccording to the present invention was applied retrospectively to theintracranial pressure curves before and after surgery, it was found amarked and significant reduction of number of intracranial pressureelevations. The matrix 35 of numbers of intracranial pressure elevationsof different levels 36 and duration's 37 before and after surgery ispresented in both Table 1 and FIG. 6. In FIG. 6 both the intracranialpressure curve and the corresponding matrix 35 of intracranial pressureelevations of different levels 36 (20-45 mmHg) and durations 37 (0.5-40minutes) are presented (before surgery at left and after surgery atright). The matrix 35 is presented as numbers during a standardisedrecording time of 10 hours 42 (actual recording time 45 before surgery 8hours and after surgery 9.85 hours). The results documents that surgeryhad a major effect in reducing the number of intracranial pressureelevations despite an unchanged mean intracranial pressure. Aftersurgery, there were no elevations of 40 or 45 mmHg, the number ofelevations of 25, 30 or 35 mmHg were markedly and significantly reduced,whereas the number of intracranial pressure elevations of 20 mmHg werenot significantly changed. For example, during a standardized recordingtime of 10 hours, intracranial pressure elevations of 30 mmHg lasting 1minute occurred 30 times before surgery (left matrix) and one time aftersurgery (right matrix). Various statistical methods may be applied tothe data to identify statistically significant changes. Accordinglyapplication of this method would have justified no re-operation in astronger and more reliable way. The patient has been followed for anobservation period of 2 years without surgery and has shown asatisfactory development in this period.

[0129] As can be seen from the above mentioned examples the inventionprovides an accurate way of comparing pressure curves. Thestandardization procedure is crucial. For example it may be useful tocompare pressures during sleep. The recording periods may be different,therefore it may be useful to standardize to a given recording time. Itmight not be representative to for example select one of 6 hours ofrecording.

[0130] In FIGS. 2-6 changes in the pressure curves of longer duration(30 seconds or above) have been illustrated. Though reference has beenmade to intracranial pressure, this represents no limitation of theinvention. Pressures from other body cavities may be presented in thesame way.

[0131] In the following FIGS. 7-10 the invention applied to single pulsepressure waves is described. Analysis of single pulse pressure wavesrepresents an even more detailed strategy for comparing pressuresbetween and within individuals.

[0132] With regard to data collection, several steps are basicallysimilar to the processes described for FIGS. 2-6. The signals from thesensor are converted to either a continuous current or voltage signalthat is further processed in the apparatus 1 or modifications thereof.The continuous current or voltage signals are converted to digitalsignals within the analogue to digital converter. Another approach is tocollect data from a vital signs monitor. Different from the datapresented in FIGS. 2-6 a higher sampling rate is required for analysisof single waves. With regard to single wave analysis the crucial pointis to have a sufficient sample rate, as well as sufficient resolutionorder to reproduce the pressure waveform properly. According to theexperience of the inventor so far a sampling rate of at least 100 Hz issufficient to find maximum and minimum values an calculate latency,amplitude and rise time for the first peak (P1) (see FIG. 7). A highersampling rate (at least 200 Hz) is required to compute the latencies andamplitudes of the second (P2) and third (P3) peaks. It is required thatthe analogue to digital converter has a resolution of at least 12 bits.It is preferably to use 16 bits or higher.

[0133] Reference now is given to FIG. 7, demonstrating the parameters ofa single pulse pressure wave that are analyzed quantitatively. Allpressure signals are recorded, usually with a recording frequency of 100Hz or above. The window with single pulse pressure waves is opened bypressing button 40 (FIG. 3). The single waves are defined by the maximum46 and minimum 47 values. By pressing another button 41 (FIG. 3), thefollowing parameters at any point on the single pressure curve may becomputed: Amplitude 48, latency 49, and rise time 50.

[0134] Latency 47 represents the time interval during which the pressureis changed from one pressure to another pressure. Each pressure signalmay be identified on the time scale because pressures are recorded alongwith a time reference. The maximum 46 and minimum values 47 identifyeach single wave. The latency from one minimum 47 value back to anotherminimum 47 value is the heart rate and the duration of the wave. Thelatency from minimum 47 to maximum 46 is the time where the pressure ofthe single wave increases from the diastolic to the systolic pressure.

[0135] People skilled in the art would know that a single intracranialpressure wave contains three peaks, the first (P1), second (P2) andthird (P3). The second peak (P2) also is termed the tidal wave and thethird peak (P3) the dichrotic wave. Whether the waveform is reproducedproperly or not depends on a sufficient resolution order and asufficient sampling rate. The expressions amplitude 48, latency 49 andrise time 50 are with reference to each of these peaks. Theidentification of the first peak (P1) is relative to maximum 46 andminimum 47.

[0136] The identification of the second peak (P2) also is relative tothe first peak (P1), and the third peak (P3) is relative to the secondpeak (P2). In the present embodiment focus is given to amplitude,latency and rise time related to the first peak (P1), though this doesnot represent any limitation of the scope of the invention. Referencesmay also be to the second (P2) and third peaks (P3).

[0137] For the first peak (P1), the amplitude ΔP1 represents therelative pressure difference between the diastolic minimum 47 andsystolic maximum 46 pressures. Latency ΔT1 is the time interval by whichpressures increase from diastolic minimum 47 to systolic maximum 46.Rise time ΔP1/ΔT1 is the quotient between difference in pressure dividedby difference in time. The differences of pressures and time representrelative values. Any type of relationship may be calculated. Thesoftware allows the calculation of a matrix 53 of number of single pulsepressure waves with pre-selected wave characteristics of differentamplitude 51 and latency 52. Any kind of combinations of single waveparameters may be computed within the matrix 53. The amplitudes 51usually are expressed in mmHg and the durations 52 in seconds.

[0138] The results may be presented as absolute numbers or aspercentages, and the results may be standardized to a selected recordingtime (for example each one minute, one hour, or even 10 hours recordingtime) 42, as compared to the actual recording period 45. During thestandardisation procedure, the numbers or percentages of single waveswith selected parameters are adjusted to a factor. The normalised timemay be chosen in each individual. An example is given. If the actualrecording time is 6 hours, and it is desired to standardise to 5 minutesrecording time, the function implies that all numbers of single wavesare divided with a factor equal to (6×60)/5 (that is 72.0).

[0139] Calculation of single pulse pressure waves automatically givesthe heart rate because each intracranial single pulse pressure wave isbuilt up from the blood pressure wave. Therefore the numbers of singlewaves with certain characteristics during a given recording time alsomay be standardized to a given heart rate 55, as compared to the actualheart rate 54. During the procedure of standardisation to a given heartrate, the heart rate must be selected beforehand. The recording intervalalso has to be selected, when an average of the heart rate must becomputed. An example is given, though this is not intended to limit thescope of the invention. It is chosen to standardise the numbers orpercentages of certain single waves to a heart rate of 60 beats aminute. Furthermore, it is chosen to average the heart rate to each5-second recording period. During this recording period of 5 seconds theaveraged heart rate is computed. Given that the total continuousrecording period is 6 hours this standardisation analysis has to berepeated a total of 4320 times (×12/minute, ×720/hour). Given that theactual average heart rate is 120 beats a second in a 5 seconds interval,the numbers or percentages of single waves during the period of 5seconds must be divided by 2, to be standardised to a average heart rateof 60 beats a second. On the other hand, if the average heart rate is 30during the 5 seconds interval the numbers or percentages of single wavesduring these 5 seconds has to be multiplied with a factor of 2, to bestandardised to a heart rate of 60 beats a second. This approach alsoallows for on-line and real-time update of standardised numbers orpercentages to a given heart rate since such an update may be performedrepeatedly every 5 seconds.

[0140] With regard to presentation of single wave parameters, a numberof variations are possible. The matrix 53 of pre-selectedcharacteristics of amplitude 51 and latency 52, may be presentedrepeatedly and comparisons between matrixes 53 at different times may beperformed. Only certain single wave parameters may be compared. Thenumbers/percentages of single wave parameters may be subject to any typeof statistical analysis.

[0141]FIG. 8 illustrates the computation of single pulse pressure waveswith certain preselected characteristics. The mathematical process ofquantitative analysis of single wave parameters may be implemented inthe software in various ways, one strategy of implementation isdescribed here. The acquired signal is first run through separatedetection of minimum 47 and maximum 46 values. The maximum thresholdvalue is set to the lowest level in the signal, and width greater thanpreselected seconds. A variety such pre-selected seconds may be chosen,and the values may depend on age. In the first studies, durations of0.1-0.2 seconds were used, but other durations may also be used. Theminimum threshold is set to highest signal level, and the width is setto pre-selected seconds, as described above. After this analysis allmaximums 46 and minimums 47 are represented with an amplitude value anda location value or time stamp. The locations are reported in indicesfrom the start of processing. This procedure will result in a lotartificial maximum and minimum detections. In other words the maximum 46and minimum 47 detection has to be refined. After this is done theresult is a collection of approved maximum and minimum pairs, which inthe next turn can be presented to the function handling the dynamicparameter analysis. First, grouping of the maximum values and minimumvalues is performed. For every maximum 46 the subsequent minimum 47 isfound. This couple makes a maximum-minimum pair. The lattermaximum-minimum pair is inspected for threshold level. The thresholdvalue has to be larger than a given value. This is performed bysubtracting the maximum amplitude and minimum amplitude. If this valueis less than the threshold value the pair is discarded. Afterwards thepair is inspected for the rise time (ΔP1/ΔT1). The rise time isexpressed as maximum amplitude minus minimum amplitude divided bymaximum location minus minimum location. This will remove pairs causedby for example an artefact in the collected signal. All rise time valueswith a value equal or larger than a given value is discarded. A largevariation is possible with regard to rise times that are discarded. Thecollection of maximums and minimum's contained now only approved values.All the dynamic values are calculated by using the approvedmaximum-minimum pairs. The values which are calculated are amplitude(ΔP1) (delta intracranial pressure expressed in mmHg) 51, latency (ΔT1)52, and rise time (ΔP1/ΔT1) 59, and heart rate 58. All these values arequite forward to find using the information found in the approvedmaximum-minimum pairs. The collections of amplitude (ΔP1) 51 values giveinformation constituting the matrix column information. The collectionsof latency (ΔT1) values 52 give the matrix row information. A matrix 53of different amplitude 51 and latency 52 combinations is computed.

[0142] An important aspect with the computation parameters of singlepulse pressure waves is that the invention computes the relativedifferences in pressures and time. These relative differences are notrelated to a zero level of pressure. Accordingly, the single waveanalysis is not influenced by the zero level of pressure, neither ofdrift of the zero level of the sensor. It should be noted that theprocedure of calculating pressure elevations of various durations FIGS.3-6 involves computation of absolute intracranial pressures (or otherpressures in a human body cavity) relative to atmospheric pressure. Theconventional methods of assessing intracranial pressure use calibrationagainst atmospheric pressure. The present invention of computation ofrelative pressures of single pressure waves solves several problems ofknown in the art.

[0143] (a) The impact of inter-individual and intra-individualdifferences in pressure is reduced. When comparing continuous pressurecurves between or within individuals, a source of error may bedifferences in the baseline pressure due to differences or drift of zerolevel. In the present invention, the accurate zero level does not affectthe single wave parameters computed.

[0144] (b) A drift in the zero level of the pressure sensor usually is aproblem with pressure sensors, particularly when pressure is monitoredcontinuously for several days. Drift in zero level of pressure has noinfluence on the single wave parameters computed as described here.

[0145] (c) The major problem with continuous monitoring of intracranialpressure by means of non-invasive sensors is the problem of determininga zero level. Thereby relative differences in pressure must be computed,but the output give non-accurate data since it may be nearly impossibleto suggest the intracranal pressure on the basis of such relativepressure assessments. In the present invention it has been possible toaccurately compute the single waves with preselected characteristics oflatency, amplitude and rise time. Since relative differences arecomputed, there is no need for a zero level. When single waves arecomputed by means of a non-invasive sensor, the present invention allowsfor determination of the intracranial pressures with a high degree ofaccuracy. On the basis of computing several hundred thousand of singlewaves and comparing the single wave parameters with the meanintracranial pressure, a high degree of correlation between amplitude,rise time and mean intracranial pressure has been found. According tothis invention, single wave analysis of signals from non-invasivesensors may both give information about relative changes in pressure andabout the intracranial pressure, as the relationships betweenintracranial pressure and single wave characteristics are knownbeforehand, on the basis of a large number of comparisons. This processmay be as follows. A non-invasive sensor 16 may be applied to thepatient and connected to the transducer 2, and the signals are processedin the apparatus 1 or modifications thereof. Such sensors 16 may useacoustic or other signals, for example by application of a sensor-deviceto the outer ear, sensing pressure in the middle ear indicative of theintracranial pressure. The signals are converted in the apparatus 1 andstored along with the time stamp. The computer software handles thedigital signals and performs the quantitative analysis of the parametersof single pulse pressure waves described here. Without knowing the exactzero level of intracranial pressures, changes of single wave parametersmay be followed continuously. This approach provides a simple approachto follow changes in intracranial pressure, and obtaining accurateinformation about the intracranial pressure.

[0146] (d) It is possible to implant permanently pressure sensors withinthe intracranial compartment, for example in conjunction withventricular shunts. Telemetric devices may record pressures. Also withthis type of pressure monitoring, drift of zero level remains a problem,hence it may be a question of whether the correct pressure is monitored.The present invention soles this problem as drift in zero level does notaffect the pressures recorded.

[0147] Exploration of the single pulse pressure waves is started bypressing the button 40, and the single wave parameters are selected bypressing the button 41. The upper figures in FIG. 8 shows the singlepulse pressure waves 57, including the time recordings 20 along the Xaxis, and pressure levels 56 along Y axis. On the Y axis the absolutepressure values are shown, it should be noted, however, that the singlepulse pressure waves are calculated by computation of relative pressureand time differences. As indicated in the upper figure to the left (FIG.8), the single waves are identified by the minimum 47 and maximum 46values. For the first peak (P1), the amplitude (ΔP1) and latency (ΔT1)are both indicated.

[0148] In FIG. 8 is also indicated the process of computing numbers ofcharacteristics of single pulse pressure waves. A graphical userinterface reveals the curve of intracranial pressure 34. A windowrevealing the pressure curve 34 along with the absolute intracranialpressure recordings 21 and the time of registration 20 is shown. Theactual recording period 45 was 472.0 seconds, and the recording periodwas not standardised 42 (0.00 in output box). During this period ofrecording the numbers of single pulse pressure waves with pre-selectedcharacteristics where computed. The amplitudes of single waves 51 wereselected to either 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5 or7.0 mmHg. The latencies of the single waves 52 were either selected to0.23, 0.25, 0.26, 0.27, or 0.28 seconds. The numbers of single pulsepressure waves with these pre-selected characteristics were computed andpresented in the matrix 53. For example, during the recording period of472.0 seconds, single pulse pressure waves with an amplitude of 5.5 mmHgand a latency 0.28 seconds (that is rise time of 5.5/0.28=19.64mmHg/sec) occurred 43 times during this recording period. The resultsmay be standardized 42 to a recording time of for example 600 seconds.In this situation all numbers must be multiplied with a factor of600/472. The numbers also may be standardised to a selected heart rate,as described above. A number of variations are possible with regard tomethods of data presentation.

[0149] The invention provides the option for comparisons of pressurecurves. For example, during monitoring of intracranial pressure duringsleep the numbers of single waves with certain pre-selectedcharacteristics may be computed (for example amplitude 4 to 8 mmHg andlatency 0.25-0.28 seconds). The numbers of such single waves may becomputed during a standardized recording period (e.g. each one minute oreach one hour) and a standardized heart rate (e.g. 70/min). The numbersof single waves may be computed within the same individual at differenttimes (e.g. before and after treatment) and compared. Alternatively thenumbers of single waves may be computed within an individual and thenumbers may be compared against a normal material.

[0150]FIG. 9 demonstrates the recordings of intracranial pressure(cerebrospinal fluid which represent one of the compartments of theintracranial cavity) 34 while infusing a liquid into the cerebrospinalspace. Pressures are presented as absolute values of mmHg on the Y axis21 and time is expressed as seconds on the X axis 20. The intracranialpressure 34 is measured simultaneously with infusion of physiologicalsaline into the lumbar cerebrospinal space, which is termed infusiontest. It is shown how the intracranial pressure curve 34 increases asliquid is infused. The increase of pressure is shown in the upperfigure. The figure also demonstrates the simultaneous computation ofrise time 59-60 and heart rate 58. With regard to rise time, twoparameters are computed simultaneously, namely ΔP1/ΔT1 59 and ΔP1/ΔT260. It is shown that the rise times ΔP1/ΔT1 59 and ΔP1/ΔT2 60 increasewith time. The heart rate 53, on the other hand, declines as thepressure increases. This illustrates that the rise times may becalculated repeatedly and plotted against time (X-axis). Alarm functionsmay be incorporated for example alarming the occurrence of ΔP1/ΔT1 59above a given level. Rise time may be an important predictor of abnormalpressure. The present technical solution allows for computation of theexact numbers or percentages of single waves with certain rise timesduring a given recording time. For example, the numbers or percentagesof single waves with a rise time between for example 10 and 30mmHg/seconds during 5 minutes of recording may be computed repeatedly,and presented graphically. FIG. 9 shows some examples of presentation ofsingle wave characteristics, though the examples represent no limitationof the scope

[0151] It should be noted in FIG. 9 that heart rate declines as the risetime increases. This is a physiological effect in the way that heartrate declines as a result of increased decompensation related toincreased pressure. Since the relative duration of each single wavecorresponds to the heart rate, the heart rate may be automaticallycomputed. The observation presented in FIG. 9 further illustrates thevalue of concomitant recording of heart rate. The parameter heart rateprovides additional information about abnormality of intracranialpressure.

[0152]FIG. 10 shows strategies to compare pressure curves. The input box40 (FIG. 3) allows for comparisons of various single pulse pressurewaves. In particular single pulse pressure waves of intracranialpressure and blood pressure may be compared, but any type of presure maybe compared. The different pressure waves are revealed simultaneouslyduring real time on-line monitoring, with the identical time reference.The output may be time on the X axis 20 and pressure 21 on the Y axis.For example, the curve of single arterial blood pressure waves 61 may berevealed simultaneously with the single intracranial pulse pressurewaves 57. For a given recording period 45 the numbers of single pulsepressure waves may be computed and the numbers may be standardised to agiven recording period 42. Furthermore the actual heart rate 54 may bestandardised to a standardised heart rate 55. The curves of single pulseintracranial 57 and arterial blood pressure 61 waves are presented inthe upper figure to the right. The time reference 20 is identical, thusallowing comparisons of single pulse pressure waves at identical pointsof time. The Y axis reveals the absolute blood pressure 62 andintracranial pressure 56 values. As described for intracranial pressure,a matrix may be computed with the opportunity to define relationshipsbetween parameters of different single waves. In the lower figure to theleft is shown a matrix 65 defining numbers of relationships between risetime for intracranial pressure waves (ΔP₁−2/ΔT₁−1) and rise time ofblood pressure waves (ΔP₁−2/ΔT₁−2). This relationship(ΔP₁−1/ΔT₁−1)/(ΔP₁−2/ΔT₁−2) has been computed and the matrix 65 presentsthe numbers by which this relationship was 1, 2, 3, or 4. This examplerepresents no limitation concerning the relationships between singlewaves that may be computed.

[0153]FIG. 11 shows different applications of the apparatus according tothe invention. As mentioned before, the invention includes a system andmethod for recording, sampling, analysis and presentation of pressurerecordings. The invention also incorporates an apparatus for highfrequency sampling of pressure recordings. It should be understood thata number of variations are possible for the system described here.Though this invention includes a portable apparatus 1 (see FIG. 1) forsampling and recording pressure signals, the computer based method maybe integrated in a number of systems and devices as indicated in FIG.11.

[0154] First, pressure signals may be derived from various pressuresensors 16 and transducers 2, either invasive or non-invasive systems.For intracranial pressure, a number of pressure sensors 16 arecommercially available. Intracranial pressure may be assessed from theepidural, subdural spaces, brain parenchyma, or from the cerebrospinalfluid space. Also for arterial blood pressure, a number of commerciallyavailable pressure sensors 16 are available. Pressure sensors 16 alsomay be implanted and recorded by means of telemetric or other devices.Noninvasive devices, utilising for example acoustic or Doppler or othersignals may record pressures. The method described here may utilizepressure signals from any source.

[0155] The computer software may be integrated in the portable apparatus1, as well as in a network station, a personal computer, medical devicecomputers 6, computer servers 6 connected to vital signs monitors, orincorporated directly in vital signs monitors. Output from thequantitative analyzes may be presented on the monitor screen, flatscreen or other devices known in the art.

[0156] Various modifications of the apparatus 1 are possible. Componentsof the apparatus 1 may be integrated in the pressure transducer 2 or invarious types of computers including medical device computers 6.

[0157] (I) In one situation the pressure signals are transmitted from acommercially available pressure sensor 16 and transducer 2 to acommercially available vital signs patient monitor. In this situationthe invention 1 may be incorporated in the vital signs patient monitor,or in a computer server that is connected to the vital signs patientmonitor, for example via network. The output of data processing may bedisplayed on a flat screen or on the vital signs patient monitor.

[0158] (II) In another situation the apparatus 1 is integrated in othercommercially devices. Such situations are described below, though theseexamples do not represent a limitation by which the apparatus may beintegrated in other systems. Commercially available pressure sensors 16may be used for assessment of intracranial pressure (e.g. Micro SensorICP Transducer coupled to Codman ICP Express; Codman & Shurtlef Inc.,Randolph, Mass.) or cerebrospinal fluid pressure (Baxter MonitoringKit). These pressure transducers 2 are connected via the portableapparatus 1 directly to a medical device computer 6. In this situationthe portable apparatus 1 is modified so that commercially availableequipments is used instead of the transducer 2. Furthermore, theportable apparatus 1 may include the galvanic element 3, signalconditioner 5, analogue to digital converter 7, central processing unit8, and eventually an output-input unit 15. Other functions such as datamemory 9, input control 10, display 12, and alarm unit 13 areincorporated in the medical device computer. In this situation, pressuresignals are transferred directly from a commercially available pressuretransducer 2 to a commercially available medical device computer 6, andthe invention 1 is used as an interface. The pressure transducer 2,interface, and medical device computer may be placed on a rack allowingthe patient to walk about, providing the opportunity to assess pressurein patients that are free to walk about.

[0159] (III) In yet another situation pressure signals are monitored andsampled by the apparatus 1 for a selected period. In this situation theapparatus 1 may be constructed in a number of ways and with a small sizeand light weight, thus allowing the patient to carry the apparatus inhis/her pocket. After the period of monitoring has been ended, theapparatus is connected to a computer and the signals analyzed andpresented.

[0160] (IV) In another situation the whole apparatus 1 is integrated ina medical device computer.

[0161]FIG. 12 illustrates graphical presentations of pressure curves 34,with accompanying matrix 53 with single wave characteristics such asamplitude 51 and latency 52. To the left is presented the pressurecurves of intracranial pressure from three different patients (casesA-C). For each pressure curve is shown the continuous pressurerecordings 34, along with the time of registration 20 and the absolutepressures 21 on the Y-axis. To the right for each pressure curve 34 isshown a histogram presentation 66 of the matrix 53 with pressureamplitudes 51 and latencies 52. For each individual case the matrix 53including amplitudes 51 and latencies 52, was standardised to astandardised recording period 42 and a standardised heart rate 55. Foreach case (A-C), the standardised recording period 42 was set to onehour. The actual recording time 45 was 6½ hours (11:30-18:00) for caseA, 10 hours (21:00-07:00) for case B, and 10 hours (22:00-08:00) forcase C. The actual heart rate 54 varied between these cases, but wasstandardised to a standardised heart rate 55 of 70 beats a minute in allthree cases. The resulting matrix 53 of numbers of single waves withcertain amplitudes 51 and latencies 52 is presented in histograms 66. Onthe Y axis 67 is shown percentage of occurrence that is how often asingle wave with a combination of a certain latency 52 and amplitude 51occurred in percentage of the total number of single waves during therecording period. On the X-axis 68 are shown the different combinationsof latency 52 and amplitude 51. For example, in these histograms, thelabel 0.14|8.50 on the X-axis refers to single waves with latency 52(ΔT₁) of 0.14 seconds and amplitude 51 (ΔP₁) of 8.50 mmHg. Accordingly,the bar 69 corresponding to the label 0.14|8.50 shows the percentage ofsingle waves with this combination occurring as percentage of totalnumber of single waves during a standardised recording time of 1 hourand a standardised heart rate of 70 beats a minute. This type ofhistogram presentation of distribution of single waves providesfundamentally new description of pressure curves. The pressure curve ofcase A is abnormal after current criteria. Mean intracranial pressurefor the whole period is 19.8 mmHg. The corresponding histogram of singlewave distribution to the left show a right orientation of the bars 69,that is a large proportion of single waves with high amplitudes 51. Incase B, on the other hand, the pressure curve is completely normalaccording to current criteria, mean intracranial pressure for the periodis 3.96 mmHg. The corresponding histogram 66 of single wavedistribution, on the other hand, shows a right-orientation similar tothat in case A. Accordingly, abnormal frequency of single waves waspresent despite a normal pressure curve according to current criteria.In case C, the pressure curve revealed a higher pressure (meanintracranial pressure 7.4 mmHg), as compared to case B (meanintracranial pressure 4.0 mmHg). However, in case C, the single wavedistribution was left orientated, with single waves with low amplitude.For example, in case C the combination on the X-axis of 0.17|2.00 (i.e.single waves with latency 52 (ΔT₁) of 0.17 seconds and amplitude 51(ΔP₁) of 2.0 mmHg) was common, but not observed in case B. Thecombination on the X-axis of 0.21|10.00 (i.e. single waves with latency52 (ΔT₁) of 0.21 seconds and amplitude 51 (ΔP₁) of 10.0 mmHg) was notobserved in case C, but was frequent in case B. The matrix 53 orhistogram 66 may be subject to further mathematical analysis todetermine the centroid (or centroidial axis) or centre of mass of thesingle wave combinations of latency and amplitude.

[0162] Such histograms may be made after the end of pressure monitoringthat is off-line. This represents no limitation of the invention ashistograms may be computed realtime and on-line repeatedly. For example,histogram presentation may be computed repeatedly each 5 or 10 secondsor each one minute, with or without standardisation functions. Suchhistograms may be incorporated in patient monitors with bars to theright indicated by red and bars to the left indicated as blue, toincorporate alarm functions. When single wave distribution changes tothe right (i.e. amplitudes of single waves increase), actions should betaken to reduce pressure.

[0163] Instead of presenting the histograms, the balanced position ofsingle wave combinations of latency/amplitude such as centroidial axisor centre of mass may be computed and displayed. Updates each 5 secondrepresents an alternative to the conventional strategy of computing theaverage of pressure signals during for instance a 5 second interval.

[0164] Though examples are given concerning intracranial pressure,similar functions may be made for any type of pressure (blood pressure,cerebral perfusion pressure, cerebrospinal fluid pressure etc.). Whichtypes of single wave distribution that should be considered as abnormaldepends on type of pressure.

[0165] References now is given to FIG. 13, showing two differentintracranial pressure recordings in one single case. Pressures wererecorded simultaneously by means of one sensor placed within the brainparenchyma (upper curve and histogram—A) and one sensor placedepidurally (lower curve and histogram—B). An epidural placement meansthat the sensor is placed outside the dura mater, actually mimickingnon-invasive pressure monitoring since the sensor is not placed withinthe cavity in which pressure is measured. For both pressure curves 34are presented the absolute pressures 21 on the Y-axis and the time ofrecording 20 on the X-axis. It should be noted that the absolutepressures differ markedly for the pressure curve for parenchymatous (A)and epidural (B) pressures. For the upper curve (A) mean intracranialpressure was 8.9 mmHg and for the lower curve (B) mean intracranialpressure was 10.3 mmHg. The morphology of the curves also was markedlydifferent. On the other hand, the distribution of single waves wasnearly identical between intraparenchymatous (A) and epidural (B)measurements. The histogram 66 is a graphical presentation of the matrix53 of amplitudes 51 and latencies 52 of single waves, where the actualrecording time 45 of 6 hours (22:00-04:00) is standardised to astandardised recording time 42 of 1 hour, and the actual heart rate 54is standardised to a standardised heart rate 55 of 70 beats a minute.The histogram 66 shows on the Y-axis the percentage occurrence, which ishow often a single wave with certain characteristics occurs inpercentage of the total number of single waves during the recordingperiod. On the X-axis is the matrix combination. For example the labelon the X-axis of 0.38|6.50 refers to single waves with latency 52 of0.38 seconds and amplitude 51 of 6.50 mmHg. For intraparenchymatous (A)and epidural (B) pressures, the single wave distribution is nearlyidentical. These figures illustrate the following: Continuous pressurerecordings are most accurately described by the single wavedistribution. Single wave distribution may be similarly presentedwhether or not the sensor is placed within the cavity pressure ismeasured.

[0166] The invention may not exclusively be used in humans but may aswell be used in animals, both in the clinical practice and in scientificexperiments.

[0167] The invention is intended used in several groups of patients withvarious clinical problems. Some examples are given though these shouldnot be understood as limitations of the scope of the invention.

[0168] Continuous intracranial or cerebrospinal fluid pressuremonitoring according to the invention described here may be used inadults and children. (a) In children intracranial hypertension may bequestioned on the basis of hydrocephalus, craniosynostosis, pseudo-tumorcerebri and questions of. (b) In children and adults either intracranialhypo- or hypertension may be questioned on the basis of shunt failure.(c) In adults with questions of so-called normal pressure hydrocephalusintrcaranial hypertension or abnormal absorption of cerebrospinal fluidmay be questioned. (d) In individuals in the intensive care unit, avital aspect is to follow abnormal changes in intracranial and bloodpressures.

[0169] Continuous blood pressure monitoring according to presentinvention may be used in (a) assessment of blood pressure medications,and in (b) children and adults in the intensive care unit in whomcontinuous blood pressure monitoring is used as part of the patientmonitoring.

[0170] Though focus is given to intracranial pressure (includingcerebrospinal fluid pressure), blood pressure, and cerebral perfusionpressure, any type of pressure in a human body cavity may be assessedaccording to the invention described here.

[0171] In all cases the invention described here may be used in (a)on-line monitoring of pressures revealing real-time changes in pressurecharacteristics, and (b) assessment of pressure curves after the end ofpressure monitoring, that is off-line.

[0172] The following examples are intended to illustrate variousapplications of the present invention, and are not intended to limit thescope thereof.

[0173] When the invention is used in children or adults in whom there isa question of abnormally increased or reduced intracranial pressureand/or blood pressure, the procedure may be as follows. In a minorsurgical procedure, the pressure sensor 16 is implanted within the skullof the patient. During the procedure, a small opening is made in the inthe skull with a subsequent small opening in the dura. The sensor iscanalled subcutaneously to the surgical opening. The sensor 16 iscoupled to the transducer 2 and then to the apparatus 1 and calibratedagainst atmospheric pressure by means of the Input control 10. Then thesensor 16 may be penetrated about one centimeter in the brainparenchyma. The surgical opening is closed and the sensor is fastened tothe skin by sutures or by other means. By means of the Input control 10,the frequency of pressure sampling is selected. The patient should bebed-ridden the first 3-4 hours after the surgical procedure, but maythen stand up and walk around, carrying the apparatus on the body. Priorto this procedure, it should be controlled that the battery is chargedand that the apparatus 1 has enough memory capacity. Otherwise the alarmfunctions 13 will inform the patient/physician. During continuouspressure recordings, the patient may move freely around. The Inputcontrol 10 contains a small keyboard with some functions that may eitherbe controlled by the patient or the nurse. This control may indicateevents such as walking around, sitting, sleeping, painful procedure thatin turn may be displayed on the intracranial pressure curve. Theintracranial pressure is monitored continuously for about 24-48 hrs.Then the apparatus is disconnected from the sensor. The sensor isremoved from the patient (a procedure that does not require localanesthesia). The physician may connect the apparatus 1 to his or herpersonal computer 6 or network station via the serial port 11. Thedigital pressure data is transferred from the memory 9 of the apparatusto either the hard disk, zip drive or network area for storage. Then thesoftware program described above may analyse the data. The intracranialpressure curve may be analyzed as described previously. As describedabove, the apparatus may have two channels allowing simultaneousrecordings of intracranial pressure and blood pressure. Blood pressurerecordings are sampled, stored and analyzed in the same way asintracranial pressure recordings. In these cases analysis andpresentation usually is performed after the end of pressure monitoring.Usually it may be useful to compare pressures during sleep and the awakestate. During the awake state it is important to differentiate whetherpressure is monitored while the patient is lying in the bed or standingup. It may be useful to record changes in pressure from lying positionto standing position or vice versa.

[0174] As described in FIG. 11 various modifications of this proceduremay be done. The pressure transducer may be connected directly to thevital signs patient monitor and the pressure signals may be transferredvia a network solution to another server or to personal computers.Alternatively, modifications of the apparatus may be used as aninterface between the pressure transducer and the computer. Though aninvasive method of recording pressures is described here various typesof non-invasive sensors may be used.

[0175] In one example a patient is transferred to the hospital with anintracranial haemorrhage and question of increased intracranialpressure. If the hospital does not have the ability to install aninvasive sensor, it may be useful to use a non-invasive sensor. Anon-invasive pressure sensor sensing pressure via the outer ear canalmay be coupled to the pressure transducer 2 and then to the apparatus 1or modifications thereof. The software according to the inventionhandles the digital signals, intracranial pressure then may be presentedin various ways.

[0176] The invention may also be used in children and adults treatedwith extracranial shunts in whom there are a question of shuntmalfunction. It is well known that over-drainage and under-drainage maygive similar symptoms that only may be properly diagnosed byintracranial pressure monitoring. A major advantage with the presentinvention is that intracranial pressure monitoring may be performed inpatients that are moving freely around. In these cases intracranialpressure monitoring in bed-ridden patients does not give reliableresults. The invention also may be used in conjunction with sensorspermanently implanted within the cranial compartment, such asventricular shunt systems.

[0177] The invention may also be used in adult patients with so-callednormal pressure hydrocephalus. This syndrome includes dementia, unsteadygait as well as urinary incontinence, which often is associated withincreased ventricles within the brain. A major problem so far has beento select the best candidates for surgery as the treatment (commonlyextracranial shunting of the ventricular fluid to the peritoneum) has arisk and treatment is non-successful in many patients. In these patientsintracranial pressure monitoring has not received widespread use due tothe limited prognostic value. The methods used so far to analyze theintracranial pressure curves of these patients have been less accurate,as previously described. The present invention provides at least twoadvantages: Continuous intracranial pressure monitoring by means of aportable apparatus 1 represents a more physiological situation than withthe patient only bed-ridden. A large number of intracranial pressurerecordings may be sampled and stored by the apparatus. Second, the newapparatus and method provide a far more accurate assessment of theintracranial pressure recordings than the currently used methods. Insuch cases comparisons of continuous pressure recordings may be donebetween individuals and within individuals (before and after treatment).In these cases particularly intracranial pressure and cerebrospinalfluid pressure is of interest. In the assessment of normal pressurehydrocephalus, infusion tests also have been shown to be of value.During infusion tests pressure is measured within the cerebrospinalfluid space, either in the lumbar spinal cord or within the cerebralventricles. The change in pressure also may be measured simultaneouslywith infusion of a liquid such as physiological saline. The presentinvention allows for calculation of single waves during the infusiontest. The applicant has shown that changes in the infusion test are mostaccurately revealed by calculation of single wave parameters. Duringinfusion testing, pressure within the cerebrospinal fluid space isrecorded, a pressure transducer for assessment of liquid pressure isused. The pressure transducer is connected to a computer. In thissituation the invention 1 may either be in the form of an analogue todigital converter. The software may be integrated within the computerallowing sampling, analysis and presentation of the data.

[0178] When pressures are measured in the cerebrospinal fluid duringso-called infusion testing, a catheter is applied to the cerebrospinalfluid space, usually either within the cerebral ventricles or to thelumbar cerebrospinal fluid space. The catheter is connected to acommercially available sensor for sensing pressures within a liquid.This pressure sensor 16 may be connected via the apparatus 1 describedhere to a commercially available computer, or via a vital signs monitorto the computer. In this situation the apparatus 1 is modified, thusserving as an interface between the sensor and the computer. Pressurerecordings are made while a fluid is infused to the cerebrospinal fluidspace. The applicant has shown that recordings of single pulse pressurewaves may be done simultaneously as the fluid is infused. According tothis intervention the various parameters of the single pulse pressurewaves may be calculated as well as the heart rate variability duringinfusion of liquid. Various strategies of assessing single pulsepressure waves may be performed in this situation. The distribution ofsingle waves during one minute of recording may be computed and relatedto the volume change that is known in this situation. The inventionallows for standardisation of numbers or percentages to a given heartrate and a given recording period. For example, the matrix 53 of singlewaves with various amplitudes 51 and latencies 52 may be computedrepeatedly during one minute of recording. Since the infusion rate andhence volume change is known a curve for each individual may be computedwith percentage of pre-selected single wave on Y axis and volume changeon X axis. When the curves of many individuals are known it is alsopossible to superimpose the recordings from one individual against areference curve from several individuals. It has previously not beenpossible to superimpose the intracranial pressure recordings of a singlesubject on the pressure volume or elastance curve. The present inventionmay provide a technical solution for this problem. Since any types ofsingle pulse wave parameters may be calculated by this invention, avariety of approaches may be possible.

[0179] With regard to on-line presentations, pressures (for exampleintracranial and blood pressures) may be presented by conventional meansas real-time presentation of numerical values of mean pressure or asreal-time presentations of intracranial pressure curves. The presentinvention provides a technical solution for continuous analysis andpresentation of parameters of single pulse pressure waves. For example,the numbers or percentages of a certain rise times (for example 10-20mmHg/sec) during a given recording period (e.g. 1 minutes) may becomputed repeatedly and presented on a graph. Thereby changes inpressures may be detected before the conventional methods, thusproviding early detection/warning of deterioration of pressures.

[0180] The invention also may be use to compare changes in bloodpressure before and after interventions. Comparable to the situationdescribed for intracranial pressure quantitative analysis andpresentation of continuous blood pressures may be computed. Changes innumbers or percentages of single pulse pressure wave parameters may becompared. In the assessment of treatment of blood pressure, comparisonof pressure curves before and after treatment is of interest. Singlepulse pressure wave parameters may be calculated before and aftertreatment with blood pressure medications. The invention provides adetailed approach for assessment of these treatments. It should be notedthat the invention may both be used in clinical practice and inscientific practice. Pressure may be monitored in both humans andanimals. In particular, the invention may be used in animal experimentsin which blood pressure medications are assessed.

[0181] This invention represents a new technical solution in variousaspects, which now will be commented on:

[0182] (a) The invention provides a technical solution for digitalrecording of pressures in individuals that are free to move about. Theapparatus is a minicomputer and may be powered by a rechargeablebattery. Thereby the patient may carry the apparatus. This provide amore physiological monitoring of pressures, including single pulsepressure waves. The currently available apparatuses for intracranialpressure monitoring are stationary apparatuses requiring the patient tobe bed-ridden during monitoring.

[0183] (b) The present apparatus allows for digital storing of a largenumber of intracranial and blood pressure recordings, different from thecurrently available apparatuses. The important aspect in this context ishigh frequency sampling of pressure recordings, though the inventionalso allows for low frequency sampling. Thereby single pulse pressurewaves may be calculated. The portable apparatus integrates standardcomponents known in the art, therefore the systems for pressurerecording and handling may also be integrated in various systems.

[0184] (c) This invention was primarily designed for analysis ofintracranial and blood pressure off-line that is after the end of 24-48hours continuous intracranial pressure monitoring. The currentlyavailable equipment for intracranial pressure monitoring are designedfor on-line monitoring allowing immediate interventions to modifypressure in critically ill patients in the intensive care unit. Whenassessing a continuous pressure curve off-line the problem is to definea representative part of the curve. Pressures change over time,therefore a misleading picture of the pressures may be provided bychoosing only one part of the curve. The present invention providesseveral strategies of quantitative analysis of pressure recordings.Elevations of pressure of various levels and durations are accuratelycomputed, thus providing an objective and quantitative description ofthe pressure curve. Single pulse pressure waves also are presented andanalysed quantitatively. The standardisation procedure described heremakes it possible to compare curves of different individuals, though therecording time for each individual may be different. Without thisstandardisation procedure, an alternative strategy might be to selectpressure curves of identical duration from different individuals. Thenit would be required to select one part of the curve, however, then itmight be difficult to select a representative part of the curve. Forexample, if intracranial pressure or blood pressure is recordedcontinuously in one individual twice (one recording of 7 hours and onerecording of 9 hours) and the two recordings are going to be compared,the problem is to compare representative portions of the curves. Thepresent invention provides a technical solution to this problem by meansof standardising the recordings to a given recording period. Thereby thewhole recording period may be utilised in the assessment.

[0185] (d) Though a major use with the present invention is off-lineassessment of pressure recordings, the invention may as well be used foron-line and real-time monitoring of single pulse pressure waves (bloodpressure, intracranial pressure, cerebral perfusion pressure, or otherpressures in a human body cavity). The invention provides a technicalsolution for continuous calculation and presentation of single pulsepressure characteristics. Calculation of the accurate numbers orpercentages of single pulse pressure parameters and comparisons of theseparameters at different times, provide a technical solution for earlydetection/warning of changes in pressure. An example is given. Thepresent invention allows for calculation of the exact numbers orpercentages of single pulse pressure waves with amplitude 6 mmHg andlatency 0.23 seconds (rise time 26 mmHg/sec) during one minute or 5minute recordings. Given that the presence of 60% of such waves during agiven recording period represents abnormality, it would be informativefor the physician to have a graphical presentation of repeatedcomputations of the percentage of this single pulse pressure wave. Infact, the invention allows for repeated computations of any combinationsof single pulse wave parameters. A continuous and real time computationof the numbers or percentages of certain rise times (for example 26mmHg/sec) during a given recording period represents an alternativepresentation. Accordingly, this invention provides a technical solutionfor early warning of deterioration of pressures.

[0186] (e) The quantitative algorithms and methods of assessingpressures have previously not been described. Several authors have usedmethods to explore the frequency distribution of pressure waves. Inparticular spectral analysis or Fast Fourier Transformation (or spectralanalysis) has been used. These methods are fundamentally different fromthe methods described here. The methods previously used have not gainedground in the clinic and have not been useful in daily clinicalpractice. The main advantage of the present invention is that theintracranial and blood pressure curves is presented in a very accurateway, that provide a reliable tool for examining normality and deviationfrom normality. The algorithms of assessing single pulse pressure wavesdescribed here particularly obtain this goal. For the patients describedhere, accurate information from the intracranial pressure curve isobligatory since the results have major impact on the decision for majorsurgery or not. In particular assessments of the various parameters ofthe single pulse pressure waves provide new and detailed information.

[0187] (f) The invention provides a technical solution for monitoringintracranial pressure without the problem of zero drift of pressuresensors or the problem of identifying the zero level. The quantitativemethod of analysing single pulse pressure waves utilises relativechanges in pressures and time, and therefore not is dependent on thezero level of pressures. It is well known that drift of zero level of apressure sensor represents a methodological problem, in particular withinvasive sensors. When continuous monitoring is performed over time suchas several days, drift of the zero level of the sensor may produce falsepressure recordings. This is related to the fact that such sensors arecalibrated against the atmospheric pressure. The same problem is seenwith pressure sensors that are permanently implanted, for exampleimplanted with a cerebral ventricular shunt system. These sensors mayfor example give a radio frequency signal that is recorded by atelemetric device. The present invention of signal handling eliminatesthe problem of zero drift. With regard to non-invasive sensors theproblem is to define a zero level. For intracranial pressure, theestablishment of a zero level requires calibration against theatmospheric pressure. The present invention computes the relativechanges of single wave parameters. In this situation the zero level ofpressure may not be known. By means of the present invention changes inparameters of single pulse pressure waves may be followed over timewithout the need for adjustment of zero level.

[0188] (g) The present invention provides a technical solution forcomparisons of pressure curves within a body cavity, that is comparisonsof waves in a wide sense of the word. Examples are comparisons ofcontinuous pressure recordings within a single subject at differenttimes, such as comparisons during a continuous monitoring of pressures.Alternatively continuous pressure recordings may be compared atdifferent times, such as before and after treatment. Pressure curves maybe compared between individuals or continuous pressure curves from anindividual may be compared against a reference material. For example,continuous intracranial pressure is monitored for 12 hours in a singlesubject. The numbers of single pulse pressure waves with pre-selectedcharacteristics concerning latency and rise time are computed. Sinceselection of only one portion of the curve would reduce the accuracy ofthe recordings, the numbers or percentages of the whole recording periodmay be standardised to a selected recording period. For example, thenumbers or percentages of single waves with certain amplitudes andlatencies during the actual recording period of 12 hours may bestandardised to numbers of waves during one hour of recordings. Thisapproach takes away the inaccuracy of selecting only one portion of thecurve. In addition to computing the quantitative characteristics of highfrequency fluctuations in pressure, quantitative analysis of the lowfrequency fluctuations in pressure may be computed, providing a morecomplete picture of the pressures. For low frequency pressure changesthe normal distribution of pressure elevations of 20 mmHg lasting 10minutes during for example one hour of recording may be computed. Due tosome individual variation in the normal distribution exact values maynot be computed but rather a distribution with the median and percentiledistribution.

[0189] (h) The invention provides a new technical solution for theclinical application of single wave analysis, when assessing continuouspressure recordings. Single pulse pressure wave parameters arecalculated quantitatively, and the numbers or percentages of certainsingle waves may be computed. The numbers/percentages may be computedduring a given recording period. Thereby the invention provides theunique opportunity to predict the placement of a continuous pressurerecording in one individual on the elastance or pressure-volume curve.It has previously not been possible to superimpose the pressurerecordings of an individual on the pressure-volume (elastance) curvebecause this curve is different for different individuals and the curvemay vary over time. The effect of this inter- and intra-individualvariation is markedly reduced by the present intervention. The presentintervention provides a tool for computing a diagram of the normalvariation of the pressure volume curve. For example the exponentialpressure volume curve originally described by Langfitt in 1966 (volumeon the X axis and pressure on the Y axis) may be presented as medianswith percentiles. The present invention provides a tool for computingthe distribution of certain single pulse pressure waves that may beconsidered as abnormal. For example, given that it is found that thepresence of a single wave with amplitude 6 mmHg and latency 0.23 secondsin 60% of the recording time is abnormal, the invention provides theoption to compute in a single patient the numbers and frequency of suchsingle waves. During infusion testing pressure changes are known alongwith changes in volume because the rate of volume change is known. Thissituation provides the opportunity to compute the distribution of thedifferent waves at different levels of the curve. For example, thedistribution of a single wave with a rise time 30 mmHg/seconds may becomputed at different pressures and volumes. During a recording time of5 minutes these single waves may constitute 20% of single waves at onepoint of the horizontal part of the curve but may constitute 80% ofsingle waves at one point of the vertical portion of the curve. Similarcomputations may be made for other single waves. Based on the recordingsof many patients, normograms may a computed. Thereby the results fromthis single subject may be superimposed on the normogram of the pressurevolume curve and an accurate description of elastance in this particularsubject is given.

[0190] (i) The present invention presents an easy-to-understandpresentation of quantitative characteristics of a pressure curve(high-frequency and low-frequency fluctuation in pressure), that is easyto understand for a physician not possessing detailed knowledge ofpressures in a human body cavity. The data processing is performed veryfast, thus not requiring time-consuming evaluation of the intracranialpressure curve.

[0191] While particular embodiments of the present invention have beendescribed herein, it is to be understood that various changes,modifications, additions and adaptations are within the scope of thepresent invention, as set forth in the following claims.

1. Method for measuring and analyzing pressure in a body cavity in apatient, comprising the steps of: a) measuring pressure by means of atleast one sensor during a period of time (hereinafter called recordingperiod) to provide at least one signal representative of the pressure,b) sampling, at selected intervals, said signal representative of saidpressure, converting the sampled signal to digital form and storing thedigital sample value along with a time reference, c) analyzing thestored sample values in order to generate a presentation of at least oneof the following: c1) number of pressure elevations with any selectedcombination of level and duration, c2) number of pressure changes withany selected combination of level difference and duration of change, c3)number of single pulse pressure waves with preselected characteristicsregarding minimum, maximum, amplitude, latency and rise time or anyother single pulse wave parameter, wherein said numbers are related to atime period and wherein the analysis performed in c3) includescomputation of relative differences of pressure and time not involving azero reference level and/or computation of absolute pressures involvinga zero reference level.
 2. Method according to claim 1, wherein step a)involves implanting a sensor in a body cavity of the patient.
 3. Methodaccording to claim 1, wherein step a) involves applying a noninvasivetechnique with a sensor using acoustic or other measuring signals. 4.Method according to claim 1, where the at least one signal representsblood pressure, and other pressure signals subjected to steps c1)-c3)represent intracranial pressure, blood pressure, cerebrospinal fluidpressure, and cerebral perfusion pressure.
 5. Method according to claim1, wherein the measurements are made while the patient is free to moveabout.
 6. Method according to claim 1, wherein step c) comprises: c4)computation of the numbers of artifacts during a recording period, c5)computation of the artifact ratio, c6) exclusion of recording sequencesof sampled values and time references when the artifact ratio is above acertain level.
 7. Method according to claim 1, wherein the presentationin step comprises the steps of presenting the data in the form ofabsolute numbers, percentages or numbers per time period as follows:absolute numbers or percentages during the actual recording period,numbers or percentages during a standardized recording period (such asone minute, one hour or 10 hours) numbers or percentages standardized toa selected heart rate (for example a standardized heart rate of 60 perminute) numbers or percentages related to normative or reference data,numbers or percentages computed repeatedly during a continuous recordingperiod.
 8. Method according to claim 1, wherein the sampling rate instep b) is at least 10 Hz, and the recording period is at least 24hours.
 9. Method according to claim 8, wherein the sampling rate in stepb) is at least 100 Hz.
 10. Method according to claim 8, wherein thesampling rate in step b) is at least 200 Hz.
 11. Method according toclaim 8, wherein the recording period is at least 48 hours. 12.Apparatus for recording and storing pressure recordings from a pressuresensor applied to a patient, comprising: a first connector (4) forconnecting the apparatus to a pressure sensor, an analog-to-digitalconverter (7) for converting received pressure measurements to digitalform, processing means in communication with the analog-to-digitalconverter (8), capable of reading out samples of the digitally convertedpressure measurements and storing said measurements in a data memory (9)connected to said processing means along with a time reference, aninput/output interface (10) in communication with the processing meansand connected to a second connector (22) for connecting the apparatus toexternal computing means (6), and a power source for supplying theapparatus with power.
 13. Apparatus according to claim 12, furthercomprising: a galvanic circuit connected to the first connector forpreventing the transfer of electrical energy from the apparatus towardsthe sensor.
 14. Apparatus according to claim 13, further comprising: asignal conditioner for removing noise from the received pressuremeasurement signals.
 15. Apparatus according to claim 14, wherein saidsignal conditioner is an analog filter connected between the firstconnector and the analog-to-digital converter.
 16. Apparatus accordingto claim 15, wherein said signal conditioner is a digital filterconnected to the output of the analog-to-digital converter. 17.Apparatus according to claim 16, further comprising: an input controlfor entering control and calibration signals.
 18. Apparatus according toclaim 17, further comprising: a display connected to said processingmeans.
 19. Apparatus according to claim 12, wherein said data memoryfurther contains instructions controlling the operation of theprocessing means.
 20. Apparatus according to claim 12, furthercomprising an alarm circuit capable of generating an audible or visualalarm upon detection of low memory capacity in the data memory or lowpower capacity in the power source.
 21. Apparatus according to claim 12,wherein said data memory is a random access memory (RAM) circuit. 22.Apparatus according to claim 12, wherein said data memory is a magneticstorage device.
 23. Apparatus according to claim 12, wherein theprocessor and the analog-to-digital converter in combination are capableof sampling the received pressure measurements with a sampling rate ofat least 10 Hz.
 24. Apparatus according to claim 23, wherein thesampling rate is at least 100 Hz.
 25. Apparatus according to claim 23,wherein the sampling rate is at least 200 Hz.
 26. Apparatus according toclaim 12, wherein the processor is programmable through input controlmeans to operate with a sampling rate between a minimum sampling rateand a maximum sampling rate.
 27. Apparatus according to claim 12,wherein said data memory has a capacity which allows the storage of atleast 24 hours of continuous sampling of the received pressuremeasurements at maximum sampling rate.
 28. Apparatus according to claim12, wherein said data memory has a capacity which allows the storage ofat least 48 hours of continuous sampling of the received pressuremeasurements at maximum sampling rate.
 29. Apparatus according to claim12, comprising a plurality of input connectors and means formultiplexing pressure signals from said input connectors for thesimultaneous recording of pressure signals from more than one pressuresensors.
 30. System for the analysis of recorded pressure data,comprising a) a communication interface for receiving a set of digitalpressure sample values; b) a data memory for storing the received samplevalues along with time references; c) processing means with access tosaid data memory, capable of analyzing said sample values in order todetermine at least one of the following: c1) number of pressureelevations with any selected combination of level and duration, c2)number of pressure changes with any selected combination of leveldifference and duration of change, c3) number of single pulse pressurewaves with preselected characteristics regarding minimum, maximum,amplitude, latency and rise time, or any other single pulse waveparameter, wherein said numbers are related to a time period as follow:numbers or percentages of pressure elevations or changes or single waveparameters during an actual recording period, numbers or percentages ofpressure elevations or changes or single wave parameters during astandardized recording period (for example 1 minute or 1 hour), numbersor percentages of pressure elevations or changes or single waveparameters during a recording time with a standardized heart rate,wherein both real-time and on-line analysis/presentations are possible,d) a video interface in communication with said processing means andcapable of, in combination with the processor means, generating a visualpresentation of the result of any analysis performed on the pressuresample values together with a graphical user interface capable ofpresenting the analyzed data: repeatedly during the recording period,with comparisons of the repeatedly analyzed data, along with normativeor reference data, e) a display for displaying the generated visualpresentation; and f) input means allowing a user of the system to enterand change parameters on which said analysis and said presentationshould be based.
 31. System according to claim 30, wherein saidparameters include at least some of the following: pressure intervalsdefining a number of pressure elevations, pressure change intervalsdefining a number of pressure change step sizes, time intervals defininga number of durations, pressure wave characteristics including minimum,maximum, amplitude and latency, selection of type of analysis, andselection of presentation of numbers as absolute numbers, percentages ornumbers/percentages during a standardized recording period ornumbers/percentages with a given heart rate.
 32. Computer programproduct for controlling a computer on which is stored a set of valuesrepresenting pressure samples with a time reference, comprising programinstructions for causing the computer to perform the steps of: receivingfrom a user interface or as pre stored default values a set ofparameters on which an analysis of said set of samples should be based;analyzing said sample values in order to determine at least one of thefollowing: 1) number of pressure elevations with any selectedcombination of level and duration, 2) number of pressure changes withany selected combination of level difference and duration of change, 3)number of single pulse pressure waves with pre-selected characteristicsregarding minimum, maximum, amplitude, latency and rise time, where saidnumbers refer to a during a time period (recording period, standardizedperiod, etc.) or a standardized heart rate. generating a visualpresentation of said analysis.
 33. Computer program product according toclaim 32, stored on a computer readable medium.
 34. Computer programproduct according to claim 32, carried on a propagated signal. 35.Computer program product according to claim 32, integrated in a portableapparatus or in different systems such as medical device computers,computer servers or vital signs monitors.